Add informational story annotation to the scene DSL

[jamiesun]

What

Adds an optional story field to the scene document: a freeform narrative description (theme, mood, dramatic intent) that downstream agents can use to review the compiled music against its intended story.

title: Forest Theme
story: >-
  Calm nocturnal forest ambience for the exploration loop — soft, unhurried,
  no threat; the player should feel safe wandering under the canopy.

Design

• Informational only, like title — zero compile semantics; a scene with story produces byte-identical MIDI to the same scene without it.
• Echoed into meta.json (single-scene output and suite manifest) so the compiled artifact is self-describing for agent review pipelines.
• Does not reopen the rejected semantic Story/character field (character: {regret: 0.9}, roadmap): that ruling concerned fields with musical meaning but no deterministic compile semantics. This is a pure annotation; a boundary note in docs/roadmap.md records the distinction.

Protocol extension gates (scene-protocol.md)

• ✅ Optional; absence compiles byte-identically to the world before the field — golden SMF test unchanged even though forest.yaml now carries a story
• ✅ Happy-path E2E: story_is_informational_and_never_affects_midi_bytes (validate + with/without byte comparison)
• ✅ Failure path: validate_rejects_non_string_story_with_location
• ✅ meta.json assertions in build_full_chain_scene_to_ogg; schema output assertion
• ✅ Acceptance matrix row updated in docs/roadmap.md
• ✅ Docs: scene-protocol table, skill SKILL.md/reference.md, README example

Validation

make check fully green: cargo fmt --check, clippy -D warnings, all 74 E2E tests.