Add gamut as pinned git dependency

[justin13888]

• Add [workspace.dependencies] entries for gamut crates as { git = "https://github.com/justin13888/gamut", rev = "295c89248b9130873fe40c99529a99769c590f98" }; add crates lazily as consumed.
• Document the pin-bump procedure: one commit that only updates the hash, full test + benchmark run, CHANGELOG note.
• Raise MSRV 1.90 → 1.92 (gamut requirement; consistent with README MSRV policy).
• No code changes beyond manifests; CI must build with the dep present.