fix(vtz): invalidate dependents' cache when a file fails to compile [#2766]

[viniciusdacal]

Summary

The dev-server file-watcher loop continued inside the compile-error branch, bypassing process_file_change. If a user introduced a syntax error in utils.ts, transitive dependents kept their stale compiled-cache entries — so even after the error was fixed, dependents could still serve stale compiled code until individually re-touched. Same shape as #2764 (delete events), different trigger.

• Compile-error branch now falls through to process_file_change so the changed file and its transitive dependents are invalidated.
• The module-graph update is still skipped on error — don't commit edges scanned from broken source; last-successful edges persist until the next successful compile, which can only over-invalidate (safe).
• HMR broadcast is suppressed while compile_result.errors is non-empty: the overlay is the user-visible state, and firing Update/FullReload on every broken keystroke would make the client re-fetch a broken module, or (for entry-file edits) reload mid-error and lose in-memory state. The next successful compile drives the refetch.
• The per-change handler body was moved out of server::http's file-watcher closure into a new server::file_change_handler::handle_file_change so it's reachable from tests without spinning up start_server_with_lifecycle.

Public API Changes

None. @vertz/runtime is documented as internal; the newly-exposed pub fn handle_file_change is used by the in-tree regression test only and is not surfaced anywhere a user would reach.

Test Plan

• cargo test --all green (incl. new regression test that models page → app → hello and verifies all three cache entries are invalidated when hello.tsx is modified with a syntax error)
• Regression test has been manually verified to fail when a return; is re-introduced in the compile-error branch
• cargo clippy --all-targets --release -- -D warnings clean (pre-push hook)
• cargo fmt --all -- --check clean
• TS build-typecheck / lint / tests green (pre-push hook)
• Trojan-source check green

Review

Ran one adversarial review; all three should-fix findings (HMR broadcast on compile error, stale graph-edge comment, single-level dependent test coverage) and the brittle broken-source string nit were addressed. Review notes and resolution: reviews/fix-2766/phase-01-hmr-compile-error.md (not committed to main).

Closes #2766.

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