fix(cat): forgetting "work" deleted network, framework and coworking

[catomean]

Closes finding 9 of #563.

The containment branch compared raw substrings both ways, and MIN_FORGET_FRAGMENT_CHARS lets a four-character fact through. So "work" was contained in network, framework, coworking and homework — a user asking Cat to forget "work" lost every memory that merely spelled it.

Reproduced before fixing. Against a five-memory corpus, forgetting "work" deleted three the user never mentioned:

+ "Builds neural networks for a living"
+ "Prefers the React framework"
+ "Uses a coworking space in Zurich"

Cat then reported those three as removed — accurately, which is exactly what makes it hard to notice. The report is true; the deletion was never asked for.

This is the mirror of finding 8 (shipped an hour ago) and the worse half: that bug told someone a memory survived when it was gone, this one destroys memories they never named, irreversibly.

The fix

Containment is now word-boundaried in both directions.

Boundaries are checked by character class, not a \b regex, because \b is ASCII-only in JavaScript: it treats café as ending after caf, so "café" would match inside cafétéria while plain words behaved correctly. \p{L}/\p{N} cover the accented alphabet the tokenizer in this file already speaks.

Deliberately not touched: the stemmer

cafés → café, weekends → weekend, photography → photographer are the matches stemming exists for, and a test now pins that they still fire. This narrows raw containment only.

One of my own test expectations was wrong on exactly that point — I'd asserted café shouldn't match cafés, but the stemmer unifies them by design. Corrected the test, not the code.

Verification

6 new tests (over-match, whole-word-in-sentence, multi-word phrase, prefix-of-longer-word, accents, stemmer-still-works).

73 Cat suites (1031 tests) and type-check green — the risk when narrowing a match is under-matching, so the existing forget tests are the real check here.

#555, which the finding said to coordinate with, is closed.