perf(py): zero-copy contiguous input + GIL release for pq_encode_batch

[konjoinfinity]

Summary

• OPTIMIZATION_OPPORTUNITIES.md item 10 flagged pq_encode_batch as always copying its [N, D] vectors input with .iter().copied().collect(), even when the numpy array was already C-contiguous (the common case). Verified this was still true and fixed it: now borrows directly via .as_slice() when contiguous, mirroring the quantize_int8_batch idiom already established elsewhere in this file, and only falls back to an owned copy for the (rare) non-contiguous case.
• Also releases the GIL around the pq_encode_into call — it's already rayon-parallel internally but previously ran with the GIL held, unlike quantize_int8_batch/train which already release it.
• centroids ([M, K, sub_dim]) is left as a direct copy: it's orders of magnitude smaller than vectors for realistic N, and PQCodebook must own its centroid buffer regardless, so there's no copy to avoid there.
• While digging through the roadmap I found items 7 and 8 in the same section (search_batch_arrays, vectorized query normalization) had also already shipped but were still listed as open — corrected those too so a future pass doesn't re-chase them.

Measured (Rust-level microbenchmark of the copy step in isolation — this container has no pytest/numpy to drive an end-to-end Python benchmark): copying a [200,000, 768] f32 array (614 MB) took a stable ~220–230ms across repeated trials, vs. ~935ms for the actual pq_encode_into work at the same scale (M=96, K=256) — the removed copy is ~20–25% of total call time at this scale, and a larger fraction for smaller batches where the fixed copy cost dominates more.

Type of change

• Performance improvement
• Documentation update

Checklist

• cargo test passes locally (238 vectro_lib tests; pq_encode_into's numerics are untouched, only input ownership changed)
• cargo clippy -- -D warnings reports no errors (verified against both the project's baseline flags and the full CI gate flags: -D warnings -D clippy::pedantic -D clippy::unwrap_used -D clippy::expect_used -D clippy::panic -D clippy::todo -D clippy::dbg_macro)
• cargo fmt --check passes on the files touched by this PR
• pytest python/tests/ — not run (no pytest/numpy installed in this container); no Python source changed, only the Rust FFI binding's internals
• ruff check python/ — not applicable, no Python changed
• No hardcoded absolute paths
• No embedding/eval/model-weight files staged
• Scoped to one logical concern (the pq_encode_batch fix + doc reconciliation for the same section)
• Before/after timing included above
• No new binary format / no serialization change

Related issues

Continuation of the plan following #99 / #100 / #104 / #105 / #106 / #107.

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