…pair indices (#690)

## Summary

Applies **byte-plane (stream-split) compression** to the region
`(browser, version)` pair-index blob — the single largest bundled data
blob.

Each region's pair indices are `u16` values, but there are only ~557
distinct pairs, so the high byte is almost always `0`. Instead of
postcard-varint-encoding interleaved `u16`s per region, the codegen now
writes **all the low bytes, then all the high bytes**, then deflates.
The high-byte plane collapses to near nothing, and isolating it from the
high-entropy low byte lets deflate model each stream far better than the
interleaved varint stream did.

The reader splits the decompressed blob at `len / 2` and recombines `lo
| hi << 8` — no postcard deserialization for this blob anymore.
`PAIR_RANGES` switches from byte offsets to element offsets (cumulative
datum counts).

## Results (lossless)

| | Before | After | Δ |
|---|--:|--:|--:|
| pair-index blob | 47,602 | 44,567 | **−3,035** |
| Linux musl example binary | 782,048 | 778,528 | **−3,520** |

The binary shrinks slightly more than the blob because the `u16`
postcard decode path is dropped. macOS file size is unchanged (16 KB
page quantization swallows the sub-page win), but the `.rodata` is 3 KB
smaller — which helps consumers that link this crate into a larger
binary.

## Why this is the remaining clean win

An entropy analysis across ~15 candidate encodings showed:
- The **percentages** blob (#2) already sits at its order-0 entropy
floor — delta/raw/byte-plane/columnar/first-value-split all fail to beat
the current delta-varint.
- The pair-index byte-plane (44,567) is already *below* the order-0
symbol entropy floor (48,944) because deflate exploits cross-region
repetition, so an order-0 arithmetic coder would be worse.
- MTF, delta+zigzag, and columnar transpose all *hurt*.

Beating the current state would require an order-1+ model (BWT / range
coder / brotli) = real decoder code that a few KB of savings won't pay
for.

Verification: all 392 tests + 14 JS-fuzz proptests pass; clippy and fmt
clean; every other generated blob is byte-identical (reproducible
codegen).

🤖 Generated with [Claude Code](https://claude.com/claude-code)