[mvanhorn]

What does this PR do?

Galaxy and Architecture can fail to draw the graph when layout text contains malformed UTF-8. The layout request returns HTTP 500 with "JSON parse failed". cbm_layout_to_json allowed invalid Unicode, which copied malformed UTF-8 bytes into its output. handle_layout then parsed that output with strict yyjson_read settings and rejected it.

The layout writer now combines YYJSON_WRITE_ALLOW_INVALID_UNICODE with YYJSON_WRITE_ESCAPE_UNICODE, replacing malformed bytes with escaped U+FFFD characters and escaping valid non-ASCII text. A regression test checks strict parsing, replacement characters, valid Unicode round trips, and escaping alongside quotes, backslashes, and newlines. It also covers empty layouts, optional fields, and non-finite numeric values.

Ran ./scripts/test.sh locally on macOS; it was still building the grammar objects when it hit its 599 second time limit, so it reported no test result.
Lint (scripts/lint.sh, scripts/ci/lint-mem.sh) was not run locally.
I did not run the new test against the unfixed writer, so this PR does not yet show it failing on the base branch.

Fixes #2907

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

AI was used for assistance.

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.