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openspec/changes/enterprise-gaming-ik-solver/proposal.md

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+# OpenSpec Change: enterprise-gaming-ik-solver
+
+- **Change ID**: enterprise-gaming-ik-solver
+- **Package**: `@suaveplan/ik-solver`
+- **Category**: gaming/rigs
+- **Target path**: `gaming/rigs/ik-solver`
+- **Status**: PROPOSED
+- **Date**: 2026-04-24
+- **Reference spec**: `/mnt/s/web/suaveplan/enterprise/ik-solver.md`
+
+---
+
+## Summary
+
+Deliver `@suaveplan/ik-solver` on the genesis branch from scratch at enterprise quality. The package provides pure-math inverse kinematics solvers: two-bone analytic IK (arm/leg chains), look-at (head/eye), and multi-chain FABRIK for arbitrary-length chains. No Three.js dependency — callers pass `Vec3`/`Quat` structs and receive solved bone rotations. Genesis adds configurable iteration limits, convergence thresholds, Zod chain config schemas, structured errors, OTel solve-time histogram, and 100% coverage with property tests for geometric correctness.
+
+---
+
+## Current-state audit (genesis)
+
+Package does not exist on genesis yet — fresh build.
+
+---
+
+## Available workspace deps on genesis
+
+| Package | Role |
+|---|---|
+| `@suaveplan/error` (`workspace:^`) | `IkSolverError`; `IK.*` codes |
+| `@suaveplan/types` (`workspace:^`) | Shared primitives |
+| `zod` (`catalog:forms`) | Chain config schemas |
+| `@opentelemetry/api` (`catalog:observability`) | Solve duration histogram, iteration count gauge |
+
+---
+
+## Public API surface
+
+```ts
+// Two-bone analytic IK (arm/leg)
+function solveTwoBone(chain: TwoBoneChain, target: Vec3, hint?: Vec3): TwoBoneSolution
+
+interface TwoBoneChain {
+  root: Vec3
+  mid: Vec3
+  tip: Vec3
+  upperLength: number
+  lowerLength: number
+}
+
+interface TwoBoneSolution {
+  upperRotation: Quat
+  lowerRotation: Quat
+  reached: boolean
+}
+
+// Look-at IK
+function solveLookAt(
+  bonePosition: Vec3,
+  boneForward: Vec3,
+  boneUp: Vec3,
+  target: Vec3,
+  clampAngle?: number    // max angle in radians
+): Quat
+
+// FABRIK multi-chain
+function solveFabrik(chain: FabrikChain, options?: FabrikOptions): FabrikSolution
+
+interface FabrikChain {
+  joints: Vec3[]
+  lengths: number[]
+  target: Vec3
+  root?: Vec3           // if set, root is constrained to original position
+}
+
+interface FabrikOptions {
+  maxIterations?: number      // default 10
+  tolerance?: number          // convergence threshold, default 0.001
+  constraints?: JointConstraint[]
+}
+
+interface JointConstraint {
+  jointIndex: number
+  minAngle: number
+  maxAngle: number
+  axis: Vec3
+}
+
+interface FabrikSolution {
+  joints: Vec3[]
+  rotations: Quat[]
+  iterations: number
+  converged: boolean
+}
+
+// Utility: build chain from bone descriptors
+function chainFromBones(bones: BoneDescriptor[], indices: number[]): FabrikChain
+```
+
+### Error codes
+
+```ts
+type IkErrorCode =
+  | 'IK_CHAIN_TOO_SHORT'
+  | 'IK_DEGENERATE_CHAIN'
+  | 'IK_MAX_ITERATIONS_EXCEEDED'
+  | 'IK_INVALID_CONSTRAINT'
+```
+
+---
+
+## New additions for genesis
+
+1. **Hint vector for two-bone** — optional pole-vector hint controls elbow/knee direction.
+2. **Joint constraints in FABRIK** — hinge and cone constraints applied per iteration.
+3. **OTel** — `ik.solve_duration_ms` histogram; `ik.iterations` histogram.
+4. **Property tests** — two-bone: tip distance from target ≤ tolerance after solve; look-at: resulting forward aligns with target direction.
+
+---
+
+## File layout
+
+```text
+gaming/rigs/ik-solver/
+  package.json
+  tsconfig.json
+  vitest.config.ts
+  src/
+    index.ts
+    error-codes.ts
+    constants/index.ts
+    types/common.ts
+    types/common.md
+    solvers/
+      two-bone/
+        two-bone.ts
+        two-bone.md
+        two-bone.types.ts
+        two-bone.test.ts
+        index.ts
+      look-at/
+        look-at.ts
+        look-at.md
+        look-at.test.ts
+        index.ts
+      fabrik/
+        fabrik.ts
+        fabrik.md
+        fabrik.types.ts
+        fabrik.test.ts
+        index.ts
+    utils/
+      vec3-math.ts
+      vec3-math.md
+      vec3-math.test.ts
+  tests/
+    property.test.ts
+```
+
+---
+
+## Impact
+
+**Downstream**: `@suaveplan/r3f-animation` uses these solvers for foot IK and hand IK; `@suaveplan/animation-events` uses look-at for gaze control. Prerequisites: `@suaveplan/error`, `@suaveplan/types`.

openspec/changes/enterprise-gaming-ik-solver/tasks.md

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+# Tasks: enterprise-gaming-ik-solver
+
+## 0. Pre-flight (non-negotiable)
+
+- [ ] 0.1 Confirm clean working tree (`git status` reports nothing) and no in-flight worktrees with uncommitted work (`git worktree list` then `git status` in each)
+- [ ] 0.2 If any worktrees exist under `.claude/worktrees/`, account for each one: still-active agent, work-pending-merge, or stale-and-prune. Stale worktrees MUST be removed (`git worktree remove`) before opening a new one — code that has been lost twice on main is the failure mode this guards against.
+- [ ] 0.3 Confirm the package set in `MANIFEST.md` matches `find packages -maxdepth 3 -name package.json` count. Drift here means a prior change archived without regenerating the manifest.
+
+## Phase 1 — Scaffold
+
+- [ ] Create `gaming/rigs/ik-solver/` (`@suaveplan/ik-solver`, `0.1.0`, `sideEffects: false`)
+- [ ] `tsconfig.json`, `vitest.config.ts`
+
+## Phase 2 — Error codes & constants
+
+- [ ] `src/error-codes.ts` + `src/constants/index.ts` — `DEFAULT_MAX_ITERATIONS`, `DEFAULT_TOLERANCE`
+
+## Phase 3 — Types
+
+- [ ] `src/types/common.ts` + `common.md`
+
+## Phase 4 — Vec3 math utils
+
+- [ ] `src/utils/vec3-math.ts` — add/sub/scale/dot/cross/normalize/distance/lerp
+- [ ] Tests + docs
+
+## Phase 5 — Two-bone solver
+
+- [ ] `src/solvers/two-bone/two-bone.ts` — law of cosines analytic solution + pole hint
+- [ ] Tests (reachable/unreachable targets, degenerate chain)
+- [ ] Docs
+
+## Phase 6 — Look-at solver
+
+- [ ] `src/solvers/look-at/look-at.ts` — quaternion from-to with up-vector and angle clamp
+- [ ] Tests + docs
+
+## Phase 7 — FABRIK solver
+
+- [ ] `src/solvers/fabrik/fabrik.ts` — forward/backward pass, joint constraints, convergence check
+- [ ] Tests + docs
+
+## Phase 8 — Property tests
+
+- [ ] `tests/property.test.ts` — two-bone reach, look-at alignment, FABRIK convergence
+
+## Phase 9 — Pre-publish gate
+
+- [ ] `bunx turbo lint typecheck test --filter=@suaveplan/ik-solver` green
+- [ ] 100% coverage
+- [ ] Publish: `bun scripts/publish.ts --package @suaveplan/ik-solver`