search: proximity/NEAR search + 9 tests

nearDocs k t1 t2 returns docs where both terms occur within k positions
(unordered); candidates from the posting intersection, filtered on positional
postings. 205/205.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/search-on-sx.md

@@ -118,9 +118,13 @@ lib/search/index.sx                     lib/search/eval.sx
 - [x] snippet / highlight generation (`highlight`, `snippet`) — 12 tests
 - [x] stemming (suffix stripping) — `stem`, `stemText`, `stemTokens`, `indexStemmed`
   — 18 tests
+- [x] proximity / NEAR — `nearDocs k t1 t2` (unordered, within k positions) — 9 tests
 
 ## Progress log
 
+- **Extension: proximity/NEAR (205/205 total).** `nearDocs k t1 t2 idx` returns docs
+  where both terms occur within k positions (unordered), candidates = posting
+  intersection, filtered on the positional postings. 9 tests.
 - **Extension: stemming (196/196 total).** Deterministic English suffix stripping
   (`stem`), `stemText`/`stemTokens`, `indexStemmed`. Two haskell-on-sx gotchas: take/drop
   over a String yield char CODES not char strings (rebuild via `joinChars . map chr`),