Fix parser and canonicalizer fuzzer failures

[lukewilliamboswell]

Summary

This draft tracks correctness fixes found by randomized testing of the Roc front end.

• isolate canonicalize fuzzer worker input files and close their directory handles
• return explicit type-application regions from the formatter
• preserve multiline platform symbol-map layout across formatting passes

Current validation

• formatter Zig module tests pass
• known type-application formatter reproducer passes
• minimized platform symbol-map reproducer passes
• local parse discovery run found two remaining import-formatting cases with one shared root cause

Next steps

• fix the import alias/comment formatting failure
• continue parse and canonicalize discovery runs
• run MiniCI and final clean five-minute runs for both targets

[lukewilliamboswell]

Triage note: reran and for 5 min each with log-only output and no new crashes were found. Both runs reported 0 crashes and 0 timeouts. No new reproducible fuzzer regressions to triage in this round.