v0.6.43 — Structured validation errors (`code` + `path` alongside the message)

Added

Structured validation errors (code + path alongside the message)

• SceneValidationError now carries .issues: ValidationIssue[] — each problem is
  { code, path, message } with a stable machine code (unknown-blend, duplicate-node-id,
  unknown-timeline-label, bad-duration, …) and a path locator (nodes.box,
  timeline.beat(in)[0], camera.zoom, audio.cues[0]), so a consumer can categorize a
  failure and point a UI at the offending element instead of parsing prose.
• Propagated in-process: SceneLoadError.issues carries them across the scene's own bundled
  core (read as a plain property, since instanceof can't cross the bundle), and
  reframe compile --json failures include issues ({ ok:false, error, kind, issues? }).
  ValidationIssue is exported from @reframe/core and reframe-video/compile.
• Fully back-compat / additive: every message is byte-identical, and .problems (string[])
  • .message + the class identity are unchanged — the existing toThrow(/…/) suite passes
    untouched. Validation isn't on the render path, so goldens are unaffected.