[SandObserver]

Summary

• Route a sequence element beginning with { or [ through scalar before the key matcher, so a flow collection refuses with its line number instead of parsing into a mapping made from its brace and first key.
• Refuse an anchor or alias where a node begins rather than testing the whole trimmed line, so * and & inside a value stay text.
• Derive a compound sequence element's key column from the dash line instead of assuming indent + 2.
• Refuse a sequence written inside a sequence line, which previously read as the literal string.
• Decode double-quoted escapes in a key with the same helper a value uses.
• Chomp block scalars in foldBlock rather than trimming trailing blank lines in scan, so + keeps every trailing newline, and drop the empty string a final newline leaves behind in the row split.
• Record SKIP.UNREADABLE for a Homepage top-level group that is not a list and for a Dashy section carrying neither items nor widgets.
• Use a caller-supplied Set of taken ids directly in collector instead of copying it.
• Attach the trapFocus keydown listener to the document in the capture phase, since the trapped root holds text that cannot take focus.
• Require both ends of a backdrop click to land on the overlay before dismissing.
• Release the previous trap when openModal's focus() is armed again.
• Move the localUrl comment to the branch it describes and drop a subsumed startsWith test, a subsumed tab pattern and an unused depth parameter.

Testing
All green, except the Docker build smoke test, which has no local Docker and is left to CI. Added 11 tests for the modal in a new ui/test/modal.test.mjs, 7 for the parser refusals and block chomping, and 5 for the skips the converters now record.

Checklist

• Stays within the project constraints: one container, no runtime dependencies, no build step (see CONTRIBUTING.md).
• Tests added or updated for new logic, and the full suite, lint, and typechecks pass.
• Docs updated if behavior or configuration changed.
• CHANGELOG.md updated under Unreleased if the change is user-visible.