feat(color): minimal cross-codec CICP/ICC emission policy

[lilith]

Stacked on #17 (feat/metadata-policy). The diff here is color-only — base it on #17, retarget to main after #17 merges.

What

A minimal cross-codec color-signaling production policy in zencodec::color:

• resolve_color_emit(&SourceColor, &EncodeCapabilities, ColorPolicy) -> ColorPlan — reconciles a source's color description against a target's capabilities and returns ColorPlan { cicp: Option<Cicp>, icc: IccDisposition }.
• ColorPolicy { Compatibility, Balanced (default), Compact, Verbatim, Custom(ColorFields) } — intent presets; the same meaning whether encoding from pixels or transcoding.
• Two capability flags split the conflated notion of "CICP works": EncodeCapabilities::cicp_is_format_authority (a conformant decoder honors CICP) vs cicp_safe_sole_carrier (safe to ship CICP-only and drop the ICC — JXL today, per libjxl's want_icc=false; false for AVIF/HEIC/PNG per 2026 ecosystem support).
• IccRetention gains DropIfCicpRepresentable / DropIfCicpSafeSoleCarrier.

Behavior: under Balanced, derive CICP from an ICC and drop the redundant profile only where CICP is the format's authority and safe as the sole carrier (→ JXL); else keep the ICC (AVIF/PNG keep both). Grayscale and CMYK suppress CICP entirely (an RGB CICP over gray/CMYK pixels would recolor them) and keep the ICC.

Deliberately minimal surface

Only what current transcode (JPEG→JXL) needs. HDR/gain-map/range/rendering-intent dispositions and a warnings-channel notes list were trimmed — every type is #[non_exhaustive], so they re-land additively when a consumer needs them. No cargo feature: gating struct fields the crate constructs itself would fork resolve_color_emit's body; removal is cleaner pre-publish.

Consumers (separate PRs)

• zenjxl: wired — Metadata::cicp drives the codestream enum color; redundant ICC stripped; the decoder synthesizes an ICC back from the enum (so a profile is still present after decode). End-to-end Display-P3 round-trip test added.
• zenavif: already emits nclx from meta.cicp.
• JPEG-in-JXL (lossless recompress) preserves color verbatim via a separate jbrd path — unaffected.
• HEIC: decode-only (no encoder).

Tests

zencodec lib 466 pass (8 new color), zenjxl 48 (incl. CICP round-trip), zencodecs metadata_conformance 15 (JXL cicp promoted Gap → Ok).

[lilith]

Superseded by #21 — the unified scenario-driven encode-emit model replaces this scenario-blind resolve_color_emit surface (the color logic here is ported into emit::resolve_emit, now driven by EmitFacts and reconciled with orientation + metadata in one plan). Branch left on origin for reference.