…assifier (#24386)

`remove_unused_class` decides inline whether an unused class can be removed and what its removal leaves behind: side-effectful heritage or computed keys and present static values get extracted into the surrounding code. The next PR in this stack adds a liveness analysis that has to make the same decision at a different place and time, and the two decisions must agree exactly — a class the analysis treats as cleanly removable must not extract anything at the removal site, or the extracted expressions would reference declarations that were removed together with it.

This moves the decision into one classifier, `classify_class_removability`, which reports `Keep`, `Extracts`, or `RemovesClean`. `remove_unused_class` classifies once up front; `RemovesClean` skips the extraction walk it cannot need. The `Extracts` path still re-runs the heritage and computed-key purity checks in its extraction loop — the classifier is shared with callers that never extract, so it reports no extraction plan; that path only runs when an `Extracts` class is actually removed.

`remove_unused_expression` now also routes through `expr_has_specialized_unused_handler`, so the list of handlers that can leave residue cannot drift from the dispatch. The same analysis relies on that list to know which initializers are dropped whole. A new specialized arm added without a predicate entry becomes dead code that its author's own tests catch; a predicate entry without a dispatch arm hits an `unreachable!`.

No behavior change. Minified output and iteration counts are byte-identical on every fixture; 553 unit tests pass.

Implemented with AI assistance (Claude Code); design, review, and verification driven by the author per the repo AI-usage policy.