Skel: Slice E — Pose mirror + left/right naming conventions

[fernandotonon]

Parent epic: #554

Goal

The plumbing every other "mirror" operation needs: detect left/right bone pairs from naming conventions, expose a mirror plane, and provide the actual pose-mirror operation that the Animation epic's Pose Library (#521) and Slice D's weight-mirror need.

Scope

• Naming conventions: detect bone pairs by name patterns. Built-in:
  • _l / _r, _L / _R
  • .l / .r, .L / .R
  • Left* / Right* (Mixamo / Unity HumanIK style)
  • *_left / *_right
• Custom convention: user can add additional <pattern> / <replacement> regex pairs in settings, persisted per project.
• Mirror plane: configurable per skeleton — world X (default for character rigs), world Y, world Z, or local-to-armature plane.
• PoseMirror operations:
  • Mirror pose: for each bone pair, swap the bone TRS such that left mirrors right and vice versa.
  • Apply pose mirrored: take a saved pose (from Anim: Slice D — Pose library (named poses, blend, apply, export) #521) and apply it flipped (e.g. a "left wave" pose becomes a "right wave" when mirrored).
  • Mirror selection: select bone Hand_l → press mirror-selection → Hand_r is now selected.
• Unpaired bones: bones with no detected mirror (e.g. spine, head) are reflected across the mirror plane in-place (rotate about the plane).
• Detection diagnostic: an inspector panel showing which bones got paired with which, plus any unpaired bones. Helps users verify their rig follows the convention before relying on mirror ops.
• Hooks: this slice provides a BoneNamingMap and PoseMirror::mirror(pose, namingMap, plane) API consumed by:
  • Slice D (Skel: Slice D — Skin weight painting #558) weight-mirror.
  • Slice B (Skel: Slice B — Bone hierarchy editing (reparent / detach / attach / split / connect) #556) mirror-chain.
  • Anim epic Anim: Slice D — Pose library (named poses, blend, apply, export) #521 pose mirroring.
  • Anim epic Anim: Slice F — Animation retargeting between skeletons #523 retargeting (auto bone-mapping uses the same convention detection).

Acceptance Criteria

• All four built-in naming conventions detect correctly on fixture rigs (Mixamo, Unity HumanIK, generic _l/_r, generic .l/.r).
• Custom naming regex works on at least one user-defined convention.
• Mirror pose visually flips a one-sided gesture correctly.
• Mirror selection works.
• Detection diagnostic panel correctly lists pairs and unpaired bones.
• API consumed by Slice D's weight-mirror and Anim: Slice D — Pose library (named poses, blend, apply, export) #521's pose-mirror without re-implementing detection.
• Sentry breadcrumbs scene.skel.mirror.*.
• Headless-CI tests cover all four conventions + the mirror-pose op.

Effort

~6 days.