Add prop interaction system and throw mechanics specification

- Introduced `stickman_prop_interactor.gd` to manage prop interactions.
- Created `phase_11_prop_interactions.md` detailing the design and implementation of prop pickup, drop, and throw mechanics.
- Defined interaction states, action lifecycles, and event triggers for prop handling.
- Specified constraints for throw actions, including target positioning and velocity calculations.
- Established acceptance criteria for prop interactions and lifecycle management.

README.md

@@ -801,6 +801,18 @@ The result: `mode` stays constant for the whole walk, exactly one `arrived` fire
 
 Spec: `docs/phase_3c_editor_spec.md`.
 
+### 24. Prop Interactions and Throw Planning (Phase 11)
+
+The Sandbox Stage action queue supports one-hand prop pickup, drop, and targeted throws. In **Direct**, select a stickman and add or edit **Pick Up Prop**, **Drop Prop**, or **Throw Prop** in its action queue. A pickup selects a stage prop and a hand (`Auto`, `Left`, or `Right`); a throw captures a stage-world point.
+
+**Throw target and guide:**
+
+- The cursor point is the requested target. The effective target is clamped to **3,600 px from the stickman's root**; if the cursor is farther away, the guide ends at the capped point while still marking the cursor position.
+- The solver accounts for stage gravity and chooses a valid descending arc whose release speed does not exceed **2,100 px/s**. Upward targets are supported when reachable; gravity can reduce their reachable distance. Targets that cannot be reached within the solver's speed and flight-time limits are shown as unreachable.
+- The guide is clipped at the first prop/world collision. In open air, it ends at the selected target; the prop continues under normal physics after that point.
+
+**Queue-aware editing preview:** when editing a queued throw, the planning guide looks at earlier actions in that queue. If an earlier pickup can provide the prop, the preview uses its grip and hand and estimates the pickup approach when needed. A hint such as **“Assumes Step 2 pickup succeeds.”** identifies the assumption; uncertain preceding actions label the guide as estimated. This preview does not run the pickup or change live prop, rig, or queue state. If no held or planned prop can be resolved, the guide is replaced by a neutral marker and an informational hint rather than a misleading trajectory. At runtime, the throw is recalculated from the actual prop, actor position, and release pose.
+
 ## File format (`.stk`)
 
 Files are UTF-8 JSON, pretty-printed with tab indentation. The format is versioned and designed to remain **backward/forward compatible** — new fields can be added without breaking older files.