Optical corrections: implement CorrectionSource seam and vignetting/CA/distortion math

[justin13888]

The optics stage (pipeline stage 2) is warn-and-skip only. That is spec-correct today (rawshift 0.1.1 parses no ARW optics metadata — Sony stores it in undecoded MakerNote tags), but two things are missing for when metadata lands:

1. The seam does not exist. docs claimed a CorrectionSource trait; grep -rn CorrectionSource crates/ returns nothing. Only the OpticsMetadata presence struct exists (crates/focale-core/src/decode/mod.rs:146).
2. No correction math at all — no vignetting gain application, no lateral-CA per-channel scaling, no distortion remap. UI toggles are inert (crates/focale-core/src/params/optics.rs, crates/focale-app/src/panels.rs:33-46).

Historical spec: PRD §3 stage 2 (pinned) — corrections come exclusively from embedded raw metadata in v1; the trait is the v2 external-profile-database seam.

Acceptance criteria:

• A real CorrectionSource trait in focale-core with the embedded-metadata implementation behind it (may return nothing for all current ARW files)
• Vignetting, lateral CA, and distortion algorithms implemented, pipeline-versioned, deterministic (CPU, fixed iteration order, no fast-math)
• DNG GainMap opcodes exercised end-to-end as the first live metadata source, if exposed by rawshift
• Warn-and-skip behaviour preserved for files without metadata; toggles take effect when metadata exists
• Blocked-on note: full ARW support tracks upstream rawshift MakerNote parsing