[ARCHIVE] full color surface (pre-trim) — revival candidate for #19

[lilith]

ARCHIVE / DO NOT MERGE. Draft preserving the full pre-trim color surface in case we want it back (≈1 week horizon).

The shipping version is #19 (feat/color-signaling), which trimmed this down to the minimal surface current transcode needs. This branch keeps everything that #19 removed, so it can be cherry-picked/revived without re-deriving it:

• ColorPlan with the full field set: range: Option<SignalRange>, rendering_intent: Option<RenderingIntent>, hdr_static: HdrStaticDisposition, gain_map: GainMapDisposition, notes: Vec<ColorNote> (vs feat(color): minimal cross-codec CICP/ICC emission policy #19's {cicp, icc}).
• Enums GainMapDisposition { None, Rewrap, DropKeepSdrBase, ApplyToRecoverSdr }, HdrStaticDisposition { Keep, DropForSdr }, RenderingIntent (+from_code), ColorNote (5 variants).
• The read_icc_intent helper and the richer resolve_color_emit body that populates range/intent and pushes the warnings notes.

Revive when a consumer lands for any of: the encode-side warnings channel (notes), gain-map/HDR dispositions, or signal-range/rendering-intent carry. Because ColorPlan is #[non_exhaustive], these re-land as a non-breaking minor bump on top of #19.

Diff = #19's 5 files but with the full 505-line color.rs.

[lilith]

Superseded by #21 — the full color surface this archived is the design now (full ColorPlan with range/intent/HDR/gain-map is back, inside the unified EmitPlan). No longer needed as a revival candidate. Branch left on origin for reference.