docs(perf): publish the dedup/parallel/lazy-copy wins and limits

REQUIREMENTS §10 carries the new measured numbers and the dedup
blocking trade-off note. DEVELOPER known-limitations is rewritten to
reflect that exact-only dedup is now O(n), fuzzy-blocking is opt-in,
and column-parallelism is scaffolding for free-threaded Python.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/DEVELOPER.md

@@ -185,8 +185,18 @@ Fixture corpora: `test-cases/text-cleaner-corpus/` (21 files) · `test-cases/enc
 
 ## Known limitations
 
-- **Dedup is O(n²)** — no blocking. Works to ~50k rows. Future: partition by first letter / ZIP prefix.
-- **Single-threaded** — could benefit from `multiprocessing`.
-- **Memory-bound** — entire file loaded into pandas. Streaming reads exist but not integrated with dedup engine.
+- **Dedup pair-compare is O(n²)** for fuzzy strategies. Exact-only
+  strategies (every column uses `Algorithm.EXACT` at threshold 100)
+  now route through an O(n) groupby fast path automatically — no API
+  change. Fuzzy strategies can opt into prefix blocking via
+  `deduplicate(..., blocking_columns=[...], blocking_prefix_len=1)`
+  to partition pairs by a cheap key (trades recall for speed).
+- **Threading is opt-in for format_standardize** —
+  `StandardizeOptions.parallel_columns > 1` uses a thread pool.
+  On CPython 3.12 the GIL caps the win at roughly neutral; the
+  scaffolding is in place for free-threaded Python 3.13+.
+- **Memory-bound** — entire file loaded into pandas. Streaming reads
+  exist but not integrated with the dedup engine.
 - **No multi-sheet dedup** — each Excel sheet processed independently.
-- **Phonenumbers minimum-length** — international numbers without country codes fall back to digits-only.
+- **Phonenumbers minimum-length** — international numbers without
+  country codes fall back to digits-only.