Migrate TIFF codec to gamut-tiff and expose TIFF encode

[justin13888]

• Replace the tiff crate (tiff-decode-tiff) with gamut-tiff decode; drop the tiff dep.
• Add EncodeOptions::Tiff (new capability, free via gamut-tiff): common + compression (Deflate default); OutputFormat gains Tiff.
• Metadata blocks flow through the gamut-metadata machinery (Migrate metadata stack to gamut-metadata #19).
• Stability gate: reference Release gamut-tiff v1 gamut#187 (gamut-tiff v1).
• Depends on: Migrate TIFF structure parser from binrw to gamut-ifd #20.

[justin13888]

Audit against gamut dde9f64 found the v1 gate (gamut#187) closed but two hard gaps for this issue: decode returns Rgb8 only (no 16/32-bit/float — rawshift's current tiff-crate path decodes U16/U32/F32, so migrating now regresses) and no Deflate on either side (this issue specifies Deflate as the default encode compression). Filed upstream as visualcommons/gamut#299 and visualcommons/gamut#300. Blocked per the upstream-first policy until both land and the pin is bumped.