Polish: #11 review leftovers (closes #23)

[irobinson010]

Summary

Closes #23 — the three consciously-deferred items from the #11 review:

• HTML player head-color drift: drop heads now brightness-track exactly like the ANSI and GIF sinks (16 .hN classes computed with the shared _cell_rgb blend) instead of a fixed full-bright color.
• Floyd–Steinberg at width: the serial inner loop runs on Python lists rather than per-element NumPy indexing — 3.5× faster (0.69s → 0.20s at 1200×800, i.e. ~400-col braille), with float64 accumulation.
• Loop seam: documented — drop positions loop seamlessly, glyph identities re-roll at the wrap (reads as the rain's normal flicker, per the review's own measurement).

Stacked on #30 (end of the current chain: #27 → #29 → #30 → this).

Test plan

• 191 tests pass (head-color test updated to the brightness-tracked classes)
• Dither rewrite benchmarked and cross-checked against the previous implementation (98.6% dot agreement on random noise; differences are float64-vs-float32 rounding on borderline pixels)
• CI matrix green

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