fix(symbols): memoize ShortNamed per index generation

[devin-ai-integration]

What and why

The membership-import pruning that bounds importPrefixAvailable (#541) asks Index.ShortNamed whether any symbol under a given last-name segment is registered under a short name. That answer was recomputed on every call by walking every qualified name ending in the segment and every symbol under each, so a model with many membership imports — the Apollo 11 example has hundreds — spent ~25% of a whole-model -validate in ShortNamed, lastSeparator and map lookups (0.44 s → 0.55 s).

ShortNamed is now memoized per index generation, following the same pattern as lookupDirectChildren:

func (idx *Index) ShortNamed(name string) bool {
	idx.readSegment(name)                 // frame dependency recorded on hits too
	generation := idx.generation.get()
	// under directChildrenMu: reset caches if generation moved, return a hit
	v := idx.shortNamedScan(name)         // the previous body, unchanged
	// under directChildrenMu: store v only if the generation is still the same
}

The cache lives beside directChildrenCache and is reset with it in resetDirectChildrenCachesLocked, so any layer write (every set/del bumps the generation) invalidates it. The resolver-side ReadSegment dependency tracking is untouched.

How it was verified

• New TestShortNamedFollowsIndexGeneration: false on an empty index, true after adding part def <s> Y;, stable on repeat, false again after the document is replaced and after it is removed.
• go test -count=1 ./internal/semantic/... ./internal/check/... ./tests/resolve; go test -race ./internal/semantic/symbols ./internal/semantic/resolve; go build ./..., go vet, gofmt -l . empty; python3 scripts/changelog.py check.
• Apollo 11 -validate, 5 interleaved runs, medians: before fix(resolve): skip membership imports that cannot surface a looked-up name #541 0.43 s, develop 0.55 s, this branch 0.45 s. The many-expose view that fix(resolve): skip membership imports that cannot surface a looked-up name #541 fixed still validates in 0.02 s.

Checklist

• make test and make lint pass locally
• Tests added or updated for the change
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (compliance rows need nothing: the census is counted at docs build)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.