test harness: add -Dname to scope the corpus to specific examples

`zig build test -Dname=examples/0625-foo.sx[,examples/0626-bar.sx]` runs ONLY the
named example(s) — full repo-relative .sx paths, comma-separated (a leading `./`
is tolerated). Empty = run everything (unchanged default).

Why: a full `-Dupdate-goldens` re-runs and rewrites all ~690 snapshots, so one
flaky/host-divergent example (AOT links, cross-arch `target` examples) can clobber
a good snapshot. `-Dname` regenerates only the named example(s) and touches
nothing else. It also busts the cached test-run result — the corpus enumerates
.sx/expected files at runtime, so a bare snapshot edit alone is otherwise served
from cache.

- build.zig: new `name` option threaded onto corpus_paths.
- corpus_run.test.zig: `nameMatchesFilter` + a per-example skip in the run loop.
- CLAUDE.md: document the targeted-regen workflow under Snapshot integrity.

CLAUDE.md

@@ -492,6 +492,23 @@ Safe workflow:
 2. Only run `zig build test -Dupdate-goldens` when you've intentionally changed output (new feature, new test, changed formatting).
 3. After regenerating, review the diff (`git diff examples/expected/ issues/expected/`) to confirm no error messages or empty output were captured.
 
+**Scope a regen to specific examples with `-Dname`.** A *full* `-Dupdate-goldens`
+re-runs and rewrites all ~690 snapshots, so a single flaky/host-divergent example
+(AOT links, cross-arch `target` examples, anything that intermittently fails) can
+silently clobber a good snapshot. To capture just the example(s) you added, pass
+their full repo-relative `.sx` path(s), comma-separated — this rewrites ONLY those
+and touches nothing else:
+
+```sh
+zig build test -Dname=examples/0625-comptime-weld-struct-field.sx -Dupdate-goldens
+zig build test -Dname=examples/0625-foo.sx,examples/0626-bar.sx   # verify just these
+```
+
+`-Dname` also busts the test-run cache (the corpus enumerates `.sx`/`expected/`
+files at RUNTIME, so editing a snapshot alone does NOT force a re-run — a plain
+`zig build test` may be served a cached result). Changing `-Dname` — or any
+compiler source — forces a fresh run.
+
 ### Adding a new language feature
 
 There is no monolithic smoke file — each feature is its own focused example.