refactor: demote Partial Ragdoll to a comparison demo

[blugart-dev]

Per project direction: Partial Ragdoll is demoted from a co-equal plugin mode to the comparison demo. It was a thin wrapper over Godot's built-in PhysicalBoneSimulator3D — "what the engine already offers", not Kickback's value (the active spring ragdoll). Removing it as a mode makes the plugin's identity unambiguous and sheds the least-tested path + a chunk of surface area.

⚠️ The comparison demo's behaviour changed → wants an in-editor visual pass before merge.

Plugin — now active-ragdoll only

• KickbackCharacter: dropped Mode.PARTIAL, the _simulator / _partial_controller detection, the partial configure, and the partial branch of receive_hit (now active-only).
• kickback_plugin.gd (setup tool): a single Active-Ragdoll flow — removed the Partial preset, the auto-created PhysicalBoneSimulator3D, the partial node, and the partial report branches.
• strength_debug_hud.gd / kickback_status_panel.gd: active-only (dropped partial discovery, _draw_partial_target, the legend key, _check_simulator).
• KickbackRaycast.DEFAULT_MASK → active layer only. The active demos' shared ybot keeps an (inactive) baked simulator that KickbackCharacter no longer disables; targeting only layer 4 prevents those bones from intercepting active hits.

Demo — the new home

• Moved partial_ragdoll_controller.gd + hit_event.gd to demo/ (DEMO-ONLY; @icon + the addon icon dropped).
• demo.tscn/demo.gd reframed as "Godot's built-in ragdoll vs Kickback's active ragdoll": frees the active char's baked simulator, enables the Godot char's, and routes one both-layers raycast by collider type (RigidBody3D → Kickback facade; PhysicalBone3D → the demo controller).

Kept (general utilities the demo reuses)

SkeletonDetector.populate_physical_bones, the KickbackLayers partial-layer constant.

Docs

README, CLAUDE.md, INTEGRATION.md, issue templates updated; the "pick one mode" / "don't mix modes" guidance removed.

Validation

Clean headless import · 109/109 GUT · clean scene-smoke (0 errors) on all 8 demos incl. the rewritten demo.tscn. (The Joint3D::_body_exit_tree lines in the test run are pre-existing engine teardown noise, unrelated.)

🔬 Visual review (please)

Open demo.tscn: left = Kickback active ragdoll, right = "Godot built-in ragdoll". Shoot each (1–5 weapons) — the left staggers/ragdolls/recovers; the right does a raw PhysicalBoneSimulator3D ragdoll then blends back. Confirm both react and the contrast reads clearly.

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