feat: Implement Phase 6a Airborne Mechanics and Landing Absorption

- Added new scene file `test_fall.scene` for testing airborne dynamics.
- Created architecture proposal in `PHASE_6a_AIRBORNE_LANDING.md` detailing airborne momentum, procedural air pose, and landing impact absorption mechanics.
- Developed specifications in `phase_6a_airborne_landing.md` outlining implementation decisions, targets, and static contracts for the airborne system.
- Introduced focused regression tests in `test_phase6a_airborne_landing.gd` to validate airborne pose, momentum, landing timeline, and queue behavior.
- Ensured all tests pass with 100% assertion success.

docs/tech_debt_and_optimizations.md

@@ -37,6 +37,7 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 25  | **Contact direct-state timing fallback** — `PropBlock` may receive `body_entered` before a matching direct-state contact row is available. | Low | Open | `_on_body_entered()` uses the documented center-direction / prop-position fallback when the row is absent, so the signal remains safe but the normal and position are less precise on that edge. (2026-09-12) |
 | 26  | **Impact speed is a coarse magnitude** — collision reactions currently scale from `prop.linear_velocity.length()`, not normal-relative velocity or an impulse computed from contact mass. | Low | Open | This keeps the signal contract simple and deterministic. Use the normal component and/or contact impulse if reaction strength needs to reflect glancing impacts accurately. (2026-09-12) |
 | 27  | **Post-IK reaction timing is engine-order-sensitive** — bounded flinch offsets are applied at the end of `StickmanRig._physics_process()`, after the IK/head-pin work. | Low | Open | The current slot is verified and prevents cumulative rotation, but future changes to Skeleton2D modification ordering could overwrite or delay the visible reaction. Revisit only with a targeted engine-order regression. (2026-09-12) |
+| 28  | **Airborne collision sweep cost scales per proxy** — Phase 6a tests the horizontal movement against each of the rig's ten synchronized collision proxies every airborne physics frame. | Low | Open | The per-proxy `PhysicsShapeQueryParameters2D.cast_motion()` checks reuse the existing proxy shapes and avoid a second body-size approximation; this is acceptable for the current sandbox scale. With many simultaneously falling rigs, the cost grows with `rigs × 10` casts per physics frame. Consider a combined query shape or broadphase candidate filter if airborne crowds become a target. (2026-09-17) |
 
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@@ -79,6 +80,7 @@ This document tracks known technical debt, optimization opportunities, and minor
 | 2026-09-04 | Phase 3c (Editor Tools — Action & Rule Editing) implemented: `scripts/action_registry.gd` / `trigger_registry.gd` (extensible action/trigger templates), `scripts/action_editor.gd` / `rule_editor.gd` / `queue_panel.gd` / `rule_panel.gd` + `scenes/*.tscn` shells, `scripts/waypoint_context.gd`, plus integration in `sandbox_stage.gd` (unified `CaptureKind` target-capture, "Edit Queue…/Edit Rules…" + right-click stickman/waypoint context menus, confirmation dialog, consequence-only rule editor) and `stage_director_visuals.gd` (`hit_test_waypoint_action`, waypoint edit highlight). Logged #19 (ad-hoc popup host-visibility discipline), #20 ("Add Rule" has no source in waypoint-filtered panels), #21 (drag-reorder nearest-row-center), #22 (`--check-only --script` needs a prior `--editor` rescan), and later the same day #23 (Phase 3c confirmation dialog popped over an exclusive panel — the delete/clear-all confirm paths do not hide the `QueuePanel`/`RulePanel` first). Documented the plan-§11.2 deviation (paramless `ActionEditor` pre-fill instead of a separate ragdoll/recover confirmation dialog) in `docs/phase_3c_editor_spec.md` §2 decision 5 — a design decision, so no debt row. Static-verified via headless `--editor --quit` + per-script `--check-only`; runtime F6 flows (queue/rule panel edit-delete-reorder, waypoint context menu, visual walk edit, full vs consequence-only rule editor) still need manual verification. |
 | 2026-09-11 | Phase 4c (Trigger Area Reliability) verified: `sandbox_stage.gd` `entered_area` now requires source + area instance IDs and detects visible world-AABB overlap for ANIMATED stickmen/unfrozen props, edge-triggered per area/movable pair. The prop enumeration scope bug was corrected. `tests/test_phase4c_area.gd` passed all 19 assertions, including transformed-area, leg-level overlap, wildcard/re-entry, speech consequence, stale-target, and deletion-cleanup coverage; editor verification confirmed the speech bubble fires for the configured source stickman. Logged #24 (session-only instance-ID rule references). |
 | 2026-09-12 | Phase 4.5 collision bodies verified: ten explicitly synchronized `AnimatableBody2D` proxies replace the old feet-point stickman collision poll; `PropBlock` supplies world-space contact data with shape-cast CCD; stage routing preserves rig→prop rules with per-part pair debouncing; bounded quadrant-aware flinches and PLAY→DIRECT authored-pose restoration were verified. `tests/test_phase45_collision_bodies.gd` passed 223 assertions; Phase 4b (53), Phase 4c (19), and Phase 3c (96) remained green. Reconciled #1/#5 and retained #25–#27 as intentional implementation limitations. |
+| 2026-09-17 | Phase 6a (Airborne Momentum, Procedural Air Pose, and Landing Absorption) implemented and user-verified. `StickmanRig` now carries signed ledge momentum with tuned exponential drag (`AIR_DRAG_X = 0.75`), swept horizontal proxy collision, additive free-fall pose with forward-before-lift arms, and height-driven landing absorption: falls below 80 px are soft; 80..900 px maps to 32..160 px torso compression and 270..520 px hand drop over 0.30 s. Independent foot contacts preserve ramps/stairs, while `LANDING_IMPACT` blocks primary queue progression and direct ragdoll remains an immediate preemption. `tests/test_phase6a_airborne_landing.gd` passed 24 assertions; `tests/test_phase4b_terrain.gd` passed 8 assertions. Logged #28 (per-proxy airborne sweep scaling). |
 
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