gamut-jpeg: baseline sequential emits fixed Annex K.3–K.6 Huffman tables

[justin13888]

• Baseline (SOF0) writes the T.81 Annex K.3–K.6 "typical" Huffman tables verbatim (verified at c2df11b: crates/gamut-jpeg/src/encoder.rs:621, :722-725, :783-795).
• Optimized-table construction already exists — huffman::build_optimal_table (Annex K.2) — but is wired only to the progressive path.
• Costs roughly 4–6% on every baseline stream versus libjpeg-turbo -optimize. Best reward/effort item in the JPEG set: the machinery is written, it just isn't reachable from the sequential encoder.
• Deliver: optimized DHT emission on the sequential path (two-pass, or buffer the coefficient histogram from the first pass), opt-in via a JpegEncoder builder method. Keep the current fixed-table output reachable so the frozen-bytes contract in encoder.rs still holds for existing configurations.
• Related: Implement space-efficient JPEG encoder #19, Investigate and implement quality optimizations for JPEG encoder #29.