gamut-jxl: only EFFORT reaches libjxl; expose modular-mode control

[justin13888]

• JxlEncoderFrameSettingsSetOption is called with exactly one enumerant, EFFORT (verified at c2df11b: crates/gamut-jxl/src/ffi.rs:204, :316). Every other JxlEncoderFrameSettingId is unreachable from JxlEncoder, and gamut-jxl-sys would need the enumerant transcribed before it could be set at all.
• The public knobs are therefore effort, distance/lossless, container, colour spec, orientation, bit depth and metadata — no coding-tool control of any kind.
• Deliver: modular-mode control on JxlEncoder, plus the minimal gamut-jxl-sys enumerant additions it requires. Opening the seam makes the rest of the frame-setting surface additive rather than another FFI change each time.
• Priority note: low rate-distortion value on the VarDCT photo curve — --modular 1 generally loses above distance ~0.5, and libjxl already selects modular for lossless. The motivation is parity with the modular-on variant that https://github.com/justin13888/image-evaluation already sweeps, so gamut-jxl can be compared against libjxl-encode on equal terms.
• Related: Stable Roadmap #168.