feat(core)!: replace generic primitives with gamut re-exports, keep sensor types

[justin13888]

Closes #18.

Two commits: a pin bump (b08a1a2 → dde9f64, per the documented procedure — hash-only commit, full test run) and the core-primitives migration.

What is Delivered

rawshift-core now re-exports gamut's generic image vocabulary and keeps only what gamut deliberately does not model: the sensor set (RawImage+builder, CfaPattern, XTransPattern, white_level_from_bit_depth), codec descriptors, the typed ImageMetadata model, and URational/SRational.

Deleted	Replaced by
Size	gamut_core::Dimensions re-export. Struct literals preserve old infallible semantics at existing call sites; fallible Dimensions::new available where strictness is wanted. Rect composes Dimensions; manual serde (flat x/y/width/height) + a dimensions_serde with-module (gamut#257 gap).
pixel.rs (Sample/FromF32/Rgb<S>/Rgba<S> + aliases)	gamut-core's sealed Pixel/Sample + markers (Rgb16, Rgba8, …), ImageBuf/ImageRef/PixelFormat/ColorModel. Old module was dead code — zero call sites beyond a prelude re-export. f32 stays transform-internal.
core BitDepth	gamut_color::BitDepth re-export (identical variants; gamut#260 provided Sixteen). bit_depth_serde with-module; containing types hand-write Default.
ColorSpace	ColorDescription — a CICP (H.273) code-point pair with consts SRGB, LINEAR_SRGB (working space, Default), DISPLAY_P3, REC2020, UNSPECIFIED; manual serde over the numeric code points.
core RgbImage	new rawshift-image wrapper over ImageBuf<Rgb16> + ColorDescription + exposure/crop. data field → data()/data_mut().
MetadataExtractor	ExtractMetadata (rename).

Upstream-first in action

The LINEAR_SRGB const is expressible only because TransferCharacteristics::Linear (code point 8) was missing from gamut — found during this migration's audit, filed as gamut#295, landed via gamut#297, pin bumped, then this change was written. No workaround, no shim. Adobe RGB / ProPhoto map to UNSPECIFIED by design: the CICP pair never lies about the samples; the preserved ICC profile is the authority. (The old enum's wide-gamut variants were never constructed anywhere — verified — so no behaviour changes.)

Structural strictness

The new RgbImage makes the length invariant (data.len() == w*h*3) unrepresentable-when-broken: constructors are fallible (RawError::Gamut { context, source }, new variant per the V1 error model), and the old mutate-data-then-set_size two-step — a window where dims and buffer disagreed — is replaced by atomic, validated replace_data. Misuse that used to produce silently inconsistent images is now a construction-time error.

Carve-out

IccProfile deletion rides with #19: gamut-icc has no sRGB-profile synthesis helper, and the type is fully entangled with the img-parts embed paths #19 replaces wholesale. Removing it here would create the dual path the epic forbids. Noted on both issues.

Breaking (0.x, accepted by the epic)

Size/ColorSpace/local BitDepth/pixel module/RgbImage.data field/MetadataExtractor are gone; RgbImage construction is fallible.

Verification Done

• cargo test --workspace — 368 passed, 0 failed (fixture-gated tests ignored as usual).
• cargo test -p rawshift-image --features serde — 350 passed; cargo test -p rawshift-core / --features serde — 18 / 22 passed, including new tests: CICP code-point round-trips, serde wire-format assertions ({"primaries":1,"transfer":8} for the working space), Rect serde round-trip, RgbImage length-invariant rejection, replace_data atomicity, bit_depth_serde rejection of unmodelled depths.
• cargo clippy --workspace --all-targets -- -D warnings and cargo clippy -p rawshift-image --all-targets --features full -- -D warnings — both clean (the full run caught 4 feature-gated pixel_count stragglers; fixed).
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps -p rawshift-core --features serde — clean.
• cargo build -p rawshift (facade) — clean. cargo fmt + lefthook pre-commit — green.
• Migration coverage swept by four independent passes over disjoint file sets (formats / processing+transforms / encode+export+prelude / examples+tests+benches), each reporting per-file, followed by a central compile-and-fix loop; final grep confirms no Size, rawshift ColorSpace::, set_size, with_color_space, or MetadataExtractor references remain.