Add Use Spot Overlay and Target Capture Bar functionality

- Introduced `StageUseTargetCaptureBar` class for managing user interactions with props and their use spots.
- Implemented UI elements including buttons for changing props and canceling actions.
- Added `PropUseSpotData` class for validation and serialization of prop use spots.
- Created tests for validating use spot data migration, serialization, and rule matching.
- Ensured proper handling of edge cases in use spot data, including unique IDs and finite positions.

docs/plans/phase_17_props_for_storytelling.md

@@ -1,6 +1,6 @@
 # Phase 17 — Props for Storytelling
 
-> **Status:** Accepted product brief. User approval: Phase 17a implementation and UI accepted. Phases 17b–17d remain deferred.
+> **Status:** Accepted product brief. Phase 17a is user-approved. Phase 17b implementation is present and awaiting in-editor user verification; Phases 17c–17d remain deferred.
 > **Audience:** UI/UX, architect, and implementation team.
 > **Purpose:** Product and design brief; technical contracts belong in later specifications.
 
@@ -17,7 +17,7 @@ Phase 17 is one product milestone with four ordered parts. Each part should be i
 | Part | Child-facing outcome | Priority | Status |
 | --- | --- | --- |
 | **17a — Prop roles and placement** | Arrange props behind or in front of characters; choose independent physical and interaction capabilities; build pass-through support props; select covered props in EDIT; use **Set Down** to release a held prop at a chosen stage spot. | First and highest | Implemented and user-approved |
-| **17b — Use spots** | Mark a handle, button, or lever on a placed prop, direct a stickman to use it, and respond through a rule. | Next | Deferred |
+| **17b — Use spots** | Mark a handle, button, or lever on a placed prop, direct a stickman to use it, and respond through a rule. | Next | Implemented; awaiting user verification |
 | **17c — Impact wobble** | Make a prop briefly wobble after a meaningful hit. | Next | Deferred |
 | **17d — Scene groups** | Organize and move several placed props together while editing a scene. | Last; lower priority | Deferred |
 
@@ -35,9 +35,9 @@ Behavior is expressed through independent capabilities, because a prop may suppo
 
 ### 17b — Use spots
 
-In EDIT, the child adds visible Use spots to a **placed prop instance** and moves each marker onto a handle, button, or lever. A Use spot differs from a pickup grip: the stickman reaches to the spot while the target prop stays in place. In DIRECT, the child chooses **Use**, selects the prop and intended spot, and sees where the stickman will approach and reach. If the spot cannot be reached, the stage explains the problem at the target rather than silently skipping the action.
+In EDIT, the child adds visible Use spots to a **placed prop instance** and moves each marker onto a handle, button, or lever. A Use spot differs from a pickup grip: the stickman reaches to the spot while the target prop stays in place. In DIRECT, the child chooses **Use**, selects the prop and intended spot, and sees whether the spot is reachable. If it cannot be reached, the stage explains the problem at the target rather than silently skipping the action.
 
-A successfully completed interaction emits a child-facing **Used prop** event that identifies the stickman, prop, and spot. A rule can respond to use of the whole prop or only one specific spot. The spot gets a brief, visible completion cue. Built-in results such as opening a door or toggling a light are later work; the rule system supplies story consequences in this phase.
+A successfully completed interaction emits a child-facing **Used prop** event that identifies the stickman, prop, and spot. New rules target one specific named spot; saved whole-prop (`spot_scope = "any"`) rules remain supported for compatibility. The spot gets a brief, visible completion cue. Built-in results such as opening a door or toggling a light are later work; the rule system supplies story consequences in this phase.
 
 ### 17c — Impact wobble
 
@@ -55,7 +55,7 @@ Depth lets children stage a character behind a table or in front of scenery. Ind
 
 - A child can save, reload, and replay a scene in which a fixed table is visually behind a stickman, supports a pot, and lets the stickman pass through. The pot can be picked up, set down onto the table with a smooth reach, released collision-free until that moment, and then fall onto it through physics. It can also be dropped or thrown and picked up again. PLAY reset restores the authored setup.
 - Visual depth and physical/interaction capabilities remain independent on placed copies of the same prop asset. A covered prop remains discoverable and editable in EDIT.
-- A child can place more than one Use spot on a prop instance, direct a stickman to a chosen spot, and make a rule fire for either the whole prop or that specific spot only after the interaction completes. Failed reach attempts give useful feedback.
+- A child can place more than one Use spot on a prop instance, direct a stickman to a chosen spot, and make a newly authored rule fire for that specific spot only after the interaction completes. Failed reach attempts give useful feedback. Legacy whole-prop rules continue to load and match.
 - A meaningful collision produces one readable wobble when enabled; resting contact does not produce continuous wobbling, and stopping PLAY clears the effect.
 - A saved scene group can be moved together in EDIT, opened to edit a member, and ungrouped. Members behave independently in PLAY and return to their authored arrangement on reset.
 - Older scenes and reusable `.prop` assets continue to open with sensible behavior. A child does not have to re-author a scene to use the new controls.
@@ -72,7 +72,8 @@ Deferred: dynamic Y sorting by character and prop ground points; built-in Use ef
 - `scripts/characters/stickman/stickman_prop_interactor.gd` handles pickup, Drop, Throw, and Set Down. It clears collisions while a prop is held and applies its authored release profile afterward. Set Down previews and runtime checks share the target solver; complex queued/rule sequences and changed runtime conditions remain useful regression cases.
 - `scripts/characters/stickman/stickman_locomotion.gd` and `scripts/stage/stage_physics_policy.gd` include actor-blocking prop collision bits and movement sweeps. Props set to block stickmen are not walkable platforms; terrain remains the movement surface. Keep obstruction, pause/reset, and ragdoll interactions in the regression checklist.
 - `scripts/stage/stage_selection.gd` provides `hit_test_all()` and the stage shows an **Objects here** chooser for overlapping candidates. The current hit test uses world-space AABBs rather than exact vector-shape containment, so the chooser may include an object when the pointer is inside its bounds but outside its artwork.
-- Props use fixed behind/in-front draw tiers. Dynamic ground-point Y sorting remains deferred. `scripts/props/prop_anchor_data.gd` continues to describe reusable grip anchors; 17b Use spots will be placed-instance data and must remain distinct from those grips.
+- Props use fixed behind/in-front draw tiers. Dynamic ground-point Y sorting remains deferred. `scripts/props/prop_anchor_data.gd` continues to describe reusable grip anchors; 17b Use spots are placed-instance data and remain distinct from those grips.
+- Phase 17b implementation is present. The architecture and UI contracts are documented in [Use spots: architecture](../specs/phase_17b_use_spots.md) and [Use spots: UI/UX](../specs/phase_17b_use_spots_ui.md). New Used prop rules choose one named spot; legacy whole-prop rules remain compatible. In-editor user verification is pending.
 - Stage collision events remain contact based. Phase 17c still needs a dependable hit-strength rule for a moving stickman striking a stationary prop without causing resting-contact jitter or requiring a child-facing force slider.
 - Stage multiselection can move several objects together in one edit, but it does not save persistent group membership. Phase 17d must preserve independent member identities and prop-targeted actions when adding saved groups.
 - Relevant prior context: [Phase 11 prop interactions](../specs/phase_11_prop_interactions.md) and the live [feature backlog](../ideas/feature_backlog.md).
@@ -83,6 +84,5 @@ Deferred: dynamic Y sorting by character and prop ground points; built-in Use ef
 
 The following design questions remain for the deferred parts of Phase 17:
 
-- **17b — Use spots:** How should EDIT show and target multiple instance spots without confusing them with reusable grip anchors, and how should DIRECT explain an unreachable spot?
 - **17c — Impact wobble:** What impact measure and visual treatment make a wobble consistent across stationary props, moving props, and stickmen without repeated jitter?
 - **17d — Scene groups:** What EDIT gesture makes it clear whether the child is selecting a whole group or one member? How should group membership be shown without covering the set artwork?