Language design: custom and overridable start poses

[a-baran-orhan]

Focused split of Workstream 2 from the dance UAT umbrella #91.

Goal

Let a document define a custom start pose, or override selected channels of a built-in start pose, so domain-specific starting positions are readable without an artificial first animation phase.

Tasks

• Write a small language design proposal for start-pose overrides.
• Compare at least these options:
  1. An inline override block attached to pose start = first-position
  2. A reusable named pose declaration followed by pose start = <name>
  3. A zero-duration / non-animated setup phase in the IR rather than new grammar
• Define interaction with ROM clamping, diagnostics, inherited channels, loop reset, share URLs, LSP completion, formatting, and older parsers.
• Ensure a base pose plus overrides stays visibly distinct from an animated step.
• Add parser, IR, timeline, LSP, playground, and round-trip tests for the selected design.

Suggested evaluation example: start in ballet first position with a domain-correct arm position without a visible transition from arms-at-sides.

Priority: P1 (representation and authoring).

Note: the IR already has startPoseOverrides on PosecodeIR — confirm current support and gaps before designing new grammar.