Docs: make the language spec discoverable and explain contact semantics by behavior

[a-baran-orhan]

Focused split of Workstream 1 from the dance UAT umbrella #91.

Goal

Make the formal language and authoring model discoverable, and explain the contact mechanisms by behavior rather than only grammar.

Canonical documentation structure

• Make the formal spec (spec/SPEC.md) reachable within one click from the playground, authoring guide, movement pages, and the main README authoring section.
• State explicitly that spec/SPEC.md is normative and spec/llm-authoring.md is a task-oriented, pasteable authoring guide.
• Keep the pasteable guide self-contained (an LLM cannot be assumed to open a referenced spec URL).
• Add an automated parity check for closed vocabularies, grammar keywords, ROM examples, start poses, contacts, props, and timing modes so the spec and guide cannot silently drift.
• Where practical, generate shared tables/snippets from parser protocol constants instead of hand-maintaining copies.

Explain constraints by behavior

• Add a compact comparison table for ground-lock, reach, pin, and grip (what each one does to the body).
• One minimal valid example and one invalid/conflicting example per mechanism.
• A visual/interactive example showing the same requested contact under reach vs pin.
• State in grammar, guide, and UI that cue is display-only coaching text (no effect on pose, constraints, timing, validation, or renderer).
• Add capability/limitation links near the copy-prompt action.

Acceptance

• A first-time user can find the normative spec without searching the repo.
• A first-time user can correctly predict whether pin or reach moves the body.
• No public docs imply cues influence animation.
• A test fails when the parser's closed vocabulary changes without corresponding authoring docs.

Priority: P0 (correctness and trust).