feat(jxl): expose modular-mode control on the encoder

[justin13888]

Summary

gamut-jxl reached libjxl with exactly one JxlEncoderFrameSettingId — EFFORT — so the public
encoder offered effort, distance/lossless, container, colour spec, orientation, bit depth and
metadata, and no coding-tool control at all.

This adds modular-mode control:

• gamut-jxl-sys declares JxlEncoderFrameSettingId::MODULAR (value 11), transcribed from the
  pinned libjxl v0.12.0 jxl/encode.h. JxlEncoderFrameSettingsSetOption already takes an i64,
  so no new extern declaration and no tests/version.rs symbol-table change.
• gamut-jxl gains ModularMode (Auto / VarDct / Modular, mapping libjxl's -1/0/1)
  and JxlEncoder::with_modular, shaped like the existing Effort / Orientation knobs.
• gamut-cli gains --jxl-modular auto|vardct|modular on gamut convert.

The motivation is parity with the modular-on variant that
image-evaluation already sweeps, so gamut-jxl
can be compared against libjxl-encode on equal terms — not rate-distortion (forcing modular
generally loses above distance ~0.5, and libjxl already picks modular for lossless). Opening this
seam also makes the rest of the frame-setting surface additive rather than an FFI change each time.

Three decisions worth reviewing

Auto is inert, not "auto = -1". When the mode is Auto the option is not sent at all, so a
default encoder's bytes are byte-identical to before this PR. auto_modular_is_byte_identical_to_the_untouched_default
pins that.

Lossless + forced VarDct is a typed refusal. libjxl's QueueFrame calls cparams.SetLossless()
(lib/jxl/encode.cc:471-473), which sets modular_mode = true unconditionally
(lib/jxl/enc_params.h:144-150) — so the frame setting is overridden and the request cannot be
honoured. Rather than silently emit a Modular stream, this returns
InvalidInput("JXL: lossless encoding cannot force VarDCT mode"). The check lives in
dispatch_encode, ahead of both the backend registry and the built-in tail, so every target and
every backend sees one refusal. Forcing Modular on lossless is allowed — it agrees with libjxl.

The codec-abi adapter declines a pinned mode rather than dropping it. EncodeConfig carries only
a codec id and a 0..=100 quality. Effort is deliberately not conveyed (a free speed/density choice
with no effect on decoded pixels), but the coding tool reshapes the codestream, so is_conveyable
now declines it exactly as it already declines non-default colour and orientation. The knob does
reach typed Rust backends, through a new JxlEncodeRequest::modular() (its constructor is
pub(crate), so this is additive).

recompress_jpeg (jbrd) re-packs the JPEG's own DCT coefficients and has no coding-tool choice to
make, so it ignores the setting — as it already does the other inapplicable knobs. Documented rather
than rejected, matching that path's existing behaviour.

Validation

• mise run test — pass (2505 passed, 0 failed)
• mise run lint — pass (clippy, -D warnings, --all-targets --all-features)
• mise run fmt-check — pass
• mise run check-commits — pass (no errors in 4 commits)
• mise run coverage / mise run mutants — running locally; see the comment below for results.
• Manual CLI end-to-end on a 96x96 RGB PNG:
  • gamut convert in.png auto.jxl --jxl-distance 1.0 → 2228 bytes
  • gamut convert in.png mod.jxl --jxl-distance 1.0 --jxl-modular modular → 1608 bytes, bytes differ
  • gamut convert in.png bad.jxl --jxl-modular vardct → error: invalid input: JXL: lossless encoding cannot force VarDCT mode [origin: gamut-jxl]
  • gamut convert mod.jxl back.png → decodes cleanly through jxl-rs

New tests: ModularMode value round-trip / range / default; with_modular chaining, equality
participation and the lossless+VarDCT refusal; modular_setting_changes_the_stream (forced Modular
differs from forced VarDCT and from libjxl's own choice, all three decodable by both jxl-rs and the
libjxl oracle); auto_modular_is_byte_identical_to_the_untouched_default;
forced_modular_lossy_stays_within_psnr; and, on the seam, that a pushed backend sees req.modular()
while a codec-abi backend declines a pinned mode and falls through to the built-in tail.

Since no decoder in the workspace reports whether a stream is VarDCT or Modular (JxlInfo carries no
such flag, and the oracle helper returns pixels only), the plumbing is pinned on stream bytes
differing plus decodability — the same shape as the existing effort_setting_changes_the_stream.

Risks and rollout

Additive and semver-minor. Auto is the default and provably byte-identical to the previous output,
so no existing stream changes. JxlEncodeRequest gained a field, but its constructor is pub(crate)
— external JxlCodestreamEncoder implementors are unaffected.

Out of scope

The sibling modular tuning enumerants (MODULAR_COLOR_SPACE 25, MODULAR_GROUP_SIZE 26,
MODULAR_PREDICTOR 27, MODULAR_NB_PREV_CHANNELS 29) and the float-valued
MODULAR_MA_TREE_LEARNING_PERCENT (28, which would additionally need
JxlEncoderFrameSettingsSetFloatOption declared plus a tests/version.rs row). The issue asked for
the minimal enumerant addition; these are now additive behind the seam this opens, and STATUS.md
records them.

Pre-existing issues noticed, deliberately not fixed here

Spotted while reading the crate; unrelated to this change, so left for a separate PR:

• crates/gamut-jxl/src/lib.rs:98 — the last doc line ends mid-sentence with an unterminated
  backtick: //! The crate is `#![deny(unsafe_code)]. The crate has no
  #![deny(unsafe_code)] / #![forbid(unsafe_code)] attribute at all (unlike every other workspace
  crate), which makes ffi.rs:9's #![allow(unsafe_code)] a no-op.
• crates/gamut-jxl/README.md:129 — also ends mid-sentence ("On the decode side,").

Issue

Closes #339

[justin13888]

Local gate results (the two the PR body left pending):

• mise run coverage — pass, 97.23% lines workspace-wide (gate is 80%). For the touched files: gamut-jxl/src/config.rs 100.00%, encoder.rs 98.17%, backend.rs 97.83%, abi.rs 98.21%.
• mise run mutants-diff — pass, 32 mutants tested: 31 caught, 1 unviable, 0 missed.

Full local set, all green: test (2505 passed / 0 failed), lint, fmt-check, check-commits, coverage, mutants-diff.