feat: Integrate custom prop library and sandbox functionality

- Added support for custom props in the sandbox, including loading, saving, and rendering.
- Implemented `PropLibrary` for managing custom prop entries and scanning for `.prop` files.
- Introduced `PropVisuals` for shared visual and collision representation of custom props.
- Enhanced `StageSpawner` to handle spawning of custom props with layered visuals and collision hulls.
- Created `MissingPropPlaceholder` to manage missing custom props during scene reloads.
- Updated thumbnail generation to support custom props with unique keys based on file paths.
- Added tests for custom prop lifecycle, authored visual values, and missing prop handling.
- Refactored existing code to accommodate new prop functionalities and ensure backward compatibility.

docs/tech_debt_and_optimizations.md

@@ -50,6 +50,9 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 38  | **Phase 9a preview does not establish runtime attachment parity** — the Prop Editor uses a private, non-physics, full-multi-shape rig preview with editor-only facing and z-order conventions. It intentionally proves anchor authoring and arm-IK evaluation, not Sandbox loading, collision, physics, mirroring, per-limb interleaving, or attachment lifecycle. | Medium | Open | A future runtime spec must independently validate the anchor transform against gameplay animation/physics and decide its visual representation, sockets or solved-wrist API, mirror strategy, collision/freeze behavior, per-limb draw order, release/ragdoll handling, and multi-shape fidelity. Do not treat preview acceptance tests as runtime tests. (2026-09-20) |
 | 39  | **Phase 9b prop centering is geometric AABB centering, not physical center of mass** — `Center Prop` uses the union AABB of internal polygon/line/circle points and deliberately excludes outline expansion, anchors, material mass, and collision geometry. | Low | Open | This is deterministic and appropriate for the fixed authoring origin, but a future physics-pivot workflow may need a separately designed center-of-mass or collision-bounds command. Phase 9b keeps canonical `origin` fixed/serialized, hides it from editing, never auto-centers after edits, and uses an explicit-save warning heuristic of `max(1024 px, 2 × bounds diagonal)`. Document-wide undo remains tracked separately by #37. (2026-09-20) |
 | 40  | **Phase 9c walking preview uses exclusive gait-or-manual ownership of the selected hand target** — the private AnimationPlayer owns the target while gait plays; beginning a cyan-target drag pauses the gait before pointer input takes ownership. | Low | Open | This deterministic policy prevents AnimationPlayer and manual IK from overwriting the same Marker2D in one frame. It means an author cannot add a live hand offset while the walk continues. If that becomes valuable, design a preview-only additive offset/blend layer rather than relying on process order. The gait remains an in-place, non-physics authoring preview and is not navigation/runtime parity (#38). (2026-09-21) |
+| 41  | **Phase 10 custom prop collision uses one convex hull** — the planned Sandbox prop renders every authored layer, but collision encloses all valid closed-shape points in one solid hull. | Low | Planned | User-approved in `specs/phase_10_prop_integration.md`: one fixture keeps dynamic rigid bodies stable and avoids overlapping per-shape contacts. Hollows and concavities collide as filled space; revisit compound collision only if playtesting shows material misses. `PropFile.flatten_to_payload()` remains for legacy single-shape consumers; #29 stays open until implementation lands. (2026-09-21) |
+| 42  | **Phase 10 Browse keeps external `.prop` paths as scene asset IDs** — arbitrary filesystem files are used in place rather than copied into `res://props/`. | Medium | Planned | This matches stickman Browse and avoids filename/import conflicts, but scenes depend on the external path remaining valid. A missing custom prop becomes an inert placeholder that preserves its original scene entry and rule references through later saves; restoring the file and reloading resolves it. Existing missing-stickman rule skipping is unchanged. See `specs/phase_10_prop_integration.md`. (2026-09-21) |
+| 43  | **Phase 10 thumbnail invalidation relies on path plus mtime** — custom cache keys will hash the full canonical path and include file modification time. | Low | Planned | This prevents equal basenames in different folders from sharing a PNG, while retaining cheap refresh checks. A filesystem with coarse timestamp resolution can miss two same-tick edits; content hashing is deferred unless observed. Cleanup must not remove built-in or unrelated external thumbnails. See `specs/phase_10_prop_integration.md`. (2026-09-21) |
 
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@@ -96,6 +99,7 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 2026-09-20 | Phase 9a Prop Attach specification written and refined (`specs/phase_9a_prop_attach.md`). Approved architecture: append Attach to the existing Arrange/Reshape system; additive role-based `.prop` anchors with a fixed synthesized `origin`; and an isolated Prop Editor stickman preview that uses real arm IK while showing the full authored prop. All Sandbox/custom-prop loading, attachment, sockets, physics/collision, lifecycle, and runtime tests are explicitly deferred. Logged #36 (all gameplay consumers deferred), #37 (document-wide undo remains absent), and #38 (editor preview is not a runtime-parity guarantee). Existing #29 remains open independently because the current runtime still flattens multi-shape props. |
 | 2026-09-20 | Phase 9b Prop Centering specification written (`specs/phase_9b_prop_centering.md`). Approved architecture: hide canonical origin from editable anchors while retaining it in `.prop` v1.0; draw a passive constant-screen-size `(0,0)` marker in all three modes; add an always-visible Center Prop document transaction; center the authoring camera on origin for startup/New/load/reset and preserve zoom after centering; and warn explicit saves with Center & Save / Save Anyway / Cancel when displacement exceeds `max(1024 px, 2 × bounds diagonal)`. Autosave stays silent and automatic recenter-on-edit is explicitly rejected. Logged #39 (AABB authoring center is not a future physics center of mass); #37 continues to track document-wide undo. |
 | 2026-09-21 | Phase 9c Prop Editor UX specification written (`specs/phase_9c_prop_editor_ux.md`). Approved architecture: a single Front Pose / Walk Left / Walk Right preview-motion selector with dynamic Pause/Resume; manual arm IK pauses gait before taking target ownership; modal typed anchor rename; Show Other Markers controls origin plus unselected anchors; one global Snap source; theme-driven `#262C39` inspector; container-driven unclipped chrome; simplified full-figure preview with no blue border. The preview directly plays the proven in-place gait/mirror policy and remains isolated from navigation, physics, actions, and Sandbox state. Logged #40 (exclusive gait/manual hand-target ownership). |
+| 2026-09-21 | Phase 10 custom prop integration specification written (`specs/phase_10_prop_integration.md`) after user approval of full layered visuals with one convex collision hull, direct external-path Browse, and missing-prop placeholders that preserve scene entries and rule references. The design reuses Sandbox prop freeze/restore and save flows, extends PropLibrary with a merged scan while retaining its four-template API, and gives custom thumbnails path-unique mtime keys. Logged #41–#43; #29 remains open pending implementation. |
 
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