fix: scale 4-bit PQ scan lookup tables by the table range

[jackylee-ch]

scan_4bit_simd builds the u8 quantization factor from qmax, the largest summed distance over the first 200 rows, then applies it to individual table entries. Entries are ~1/m of a sum, so the table collapses into the low 255/m of the u8 range and rows past 200 rank worse. fastscan.rs already scales by the table's own max; this reuses its quantize_distance_table.

Tests:

• new test_scan_4bit_simd_large fails before this change (row 200 off by 4.04, tolerance 2.80), passes after
• cargo test --release --workspace (503 passed)
• cargo fmt --all -- --check
• cargo +1.98.0 clippy --release --all-targets --workspace -- -D warnings