refactor: consolidate reveal state

[bnema]

Summary

• store pending and revealed lifecycle state as identity maps
• preserve identity-checked replacement, release, rebuild, stale callback, and visibility serialization paths
• add an invariant test ensuring each state entry stores its WebView identity

Validation

• /usr/bin/go test ./internal/ui/coordinator/content -count=1
• /usr/bin/go test -race ./internal/ui/coordinator/content -count=5
• /usr/bin/go fmt ./...
• make lint
• make test
• make staticcheck
• /usr/bin/go vet ./...
• make build
• make check

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✨ Finishing Touches

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• Create PR with unit tests
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