Add new stage editor scripts for target capture, terrain painting, theme management, rule engine, and spatial indexing

- Implement `StageTargetCaptureSession` for managing target capture sessions with throw and set down positions.
- Create `StageTerrainPainter` for terrain drag painting and overlap classification with stroke preview.
- Introduce `StageThemeController` to handle theme values and resource loading for the Sandbox Stage.
- Develop `StageRuleEngine` for runtime event matching and collision bookkeeping.
- Add `StageSpatialIndex` for broadphase stage lookups with advisory spatial indexing.

docs/architecture/tech_debt_and_optimizations.md

@@ -76,6 +76,7 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 64  | **Legacy scene draw order can interleave props and characters** — the new two-choice depth control cannot exactly encode an older prop between two characters. | Low | Planned | Preserve `World` order with a persisted `legacy_draw_order` marker and z-index zero until the child explicitly picks Behind or In front. Show an Old scene order cue in the card; selecting a depth applies that choice relative to every character. See `docs/specs/phase_17a_prop_roles.md`. (2026-10-05) |
 | 65  | **Large custom prop switches need state parity with native buttons** — a drawn 64 × 34 px switch inside a full-row `Button` replaces the stock small `CheckButton` icon. | Low | Planned | The proposed `PropSettingsToggleRow` should keep native focus, keyboard activation, disabled state, and one `toggled` emission per press; its child labels must ignore pointer input. A compact depth cue lives inside each choice button, while the stage remains the live preview. See `docs/specs/prop_settings_card_refinement.md`. (2026-10-09) |
 | 66  | **Stage selection panels duplicate layout and visual rules** — the prop card owns its own shell sizing, choice styles, and `stage_prop_settings` tokens, making later inspector cards prone to drift. | Low | Planned | Promote the current prop card to a small shared Stage inspector shell, choice group, and toggle row with `stage_panels` theme tokens. Keep domain handlers in `SandboxStage`; preserve the existing prop role schema. During migration, resolve old prop tokens as fallback only temporarily and remove duplicate reads once the prop card uses the shared controls. See `docs/specs/stage_panel_ui_system.md`. (2026-10-09) |
+| 67  | **SandboxStage controller is too large for comfortable Godot IDE editing** — stage UI, terrain painting, document workflow, runtime rules, and speech coordination shared one roughly 7,600-line script. | High | In Progress | Approved slices 1–16 extracted reusable stage components and reduced `sandbox_stage.gd` to 5,009 lines (about 34%); the below-3,800-line target remains unmet by 1,209 lines. Root input/selection, authored queue/rule writes, recovery application, and save/commit ordering remain coordinated there. Root `--check-only` parse passes; behavior and in-editor responsiveness checks are pending. See `docs/specs/sandbox_stage_composition_refactor.md`. (2026-10-10) |
 
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@@ -157,5 +158,7 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 2026-10-05 | Phase 17a prop roles and Set Down architecture specification written for final user sign-off (`docs/specs/phase_17a_prop_roles.md`). Retain `PropBlock` as a stable `RigidBody2D` with authored-start/held/released profiles; make Set Down a world release target with terrain-grounded reach and physics settling; use explicit contact-pair policy and a grounded walking sweep. Legacy scenes migrate with dynamic prop behavior and a saved draw-order marker. Logged #62–#64. No implementation code changed. |
 | 2026-10-09 | Proposed Prop settings card refinement written for user approval (`docs/specs/prop_settings_card_refinement.md`): full-width 64 × 34 px switches, explicit selected choice styling, plain-language copy, nearby help, compact scrollable card, and small front/behind cues. Logged #65 for custom control state parity. No implementation code changed. |
 | 2026-10-09 | Proposed Stage selection panel UI system drafted for user approval (`docs/specs/stage_panel_ui_system.md`). The current prop card is the reference; a shared shell, choice group, toggle row, and theme tokens support later Stage inspectors. Logged #66 for duplicated panel presentation. No implementation code changed. |
+| 2026-10-10 | Sandbox Stage composition refactor specified for developer handoff (`docs/specs/sandbox_stage_composition_refactor.md`). Defines state ownership, explicit component APIs, compatibility facades, and an ordered migration beginning with spatial indexing and terrain painting. Logged #67; implementation followed in the entries below. |
+| 2026-10-10 | Sandbox Stage composition refactor slices 1–16 implemented (`docs/specs/sandbox_stage_composition_refactor.md`): extracted spatial indexing, terrain painting, stage chrome and views, document/scene adapters, run/director/rule/action/speech sessions, asset browsing, theme loading, and target capture. `sandbox_stage.gd` is 5,009 lines, about 34% smaller than the approximate 7,600-line baseline; the below-3,800-line target remains open. Godot 4.7.1 root `--check-only` parse passes. No behavior tests or in-editor checks were run; #67 remains In Progress. |
 
 _End of Log_