feat(pocket3d): add reactive playable world

[doodlewind]

Summary

• add a renderer-free pocket3d-world crate with deterministic fixed turns, stable entities, bodies, sphere/capsule contacts, attachments, structure damage, heat, moisture, fuel, combustion, snapshots, ordered events, and state hashing
• add a Pocket3D orchard POC with apple trees, detachable fruit, axe cutting, pickup/drop, embers, water, fire propagation, third-person movement, camera, particles, HUD, and deterministic headless acceptance receipts
• replace the placeholder explorer with an original Blender-generated character: modeled face, hair, layered clothing, hands, boots, axe, one 19-joint skin, and named Idle, Walk, and Chop clips
• fix character interactions: apples follow the animated hand.L socket and release from that world position, Walk arms swing fore/aft without lateral flare, and the axe blade stays on the outside of the right hand through every clip
• make Q emit a short 2.8 m ballistic water burst from the character's waist; rendering, curved collision volume, distance falloff, and a conserved per-turn water budget share one deterministic trajectory
• replace fixed douse cooling and unbounded neighbour heating with shared physical laws: inlet-water sensible heat, evaporation latent heat, fuel-funded combustion energy, non-duplicating spatial transfer, and one final combustion transition per entity per turn
• add Pocket3D named-node animation sampling, in-memory self-contained GLB loading, and opt-in procedural sky, toon bands, rim light, distance fog, and inverse-transpose normal transforms
• document ownership, scaling seams, backend constraints, and the independent-research boundary; no code, assets, data, shaders, equations, or tuning are copied from zeldaret/botw

Explorer asset contract

• Blender 5.1.2 generator; clean headless regeneration is byte-identical to the checked-in GLB
• GLB SHA-256 7948c785bb7bf5df2cab1254ac5cda35d036a2b6c9ff910ae19a88f87c95dbc1
• one skinned ExplorerMesh, 14 material primitives, 5,410 exported vertices, 6,752 triangles, one skin, and 19 joints
• exact Idle / Walk / Chop clips; fresh Blender import recovered one armature, one mesh, all 19 bones, and all three actions
• Walk arm fore/aft travel 0.7032 m, lateral drift 0.0001 m, opposed-phase error 0.0 m, and planted-foot error <= 0.004 m
• axe blade minimum outward clearance 0.1961 m, minimum body-center clearance 0.5439 m, and forward Chop travel 1.4226 m
• five studio previews, including rear and side Walk views, plus editable .blend, deterministic generator, and machine-readable receipt

Validation

• cargo test --locked --manifest-path engine/Cargo.toml -p pocket3d (21/21)
• cargo test --locked --manifest-path engine/Cargo.toml --workspace -- --test-threads=1
• cargo test --locked --manifest-path engine/Cargo.toml -p pocket3d-world -- --test-threads=1 (25/25 invariant tests)
• cargo test --locked --manifest-path engine/Cargo.toml -p playable-world -- --test-threads=1 (23/23)
• cargo clippy --locked --manifest-path engine/Cargo.toml -p pocket3d-world -p playable-world --all-targets -- -D warnings
• clean Blender 5.1.2 regeneration in an isolated directory; fresh import passed and GLB digest/bytes matched
• rustfmt --edition 2024 --check on every changed Rust source; git diff --check
• Apple M3 Max / Metal headless captures for Walk, Chop, held-apple, short water-burst, active campfire dousing, and the stable extinguished state
• campfire-douse emitted and delivered one conserved 5.2799993 water budget across the flame and two logs; all three remained extinguished for 246 turns after spraying, with no re-ignition
• orchard-fire after exactly 720 fixed turns retained state eb36bdf69ea456a7; fracture, fruit terrain contact, reactive-state change, spatial fire propagation, and damp-fuel resistance all passed