fix: describe graph reference gaps without declaring live code dead

[mvanhorn]

What does this PR do?

Update FilterPanel.tsx so the section is titled "Reference coverage", the candidate count reads "N with no known inbound references", and the existing filter reads "Show only nodes with no known inbound references". Add a short explanation in that section: "No known inbound references does not prove code is unused. Static analysis may miss constructors, implicit or framework calls, and callbacks." Change the dead entry in STATUS_LEGEND in colors.ts to "No known inbound references", and clarify the nearby status comment while retaining its existing key and color.

The graph UI presents functions and methods with no known incoming CALLS, USAGE, or CALL_REFERENCE edges as definitively dead, even though constructors, runtime/framework dispatch, and callbacks can execute without those edges being extracted. The reporter found 50 false positives in 50 sampled nodes, and another user reproduced the constructor case on v0.10.8. Owner DeusData explicitly requested that the first fix remove false certainty at the presentation boundary and leave extraction improvements to separate graph-quality work. This plan implements that first step and should reference the issue with Refs #1642, rather than claiming to repair missing graph edges or close the entire report.

Fixes #1642

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  Not run: no test command resolved in this workspace, so nothing was executed to pass.
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
  Not run: no test command resolved in this workspace, so nothing was executed to pass.
• Lint passes (make -f Makefile.cbm lint-ci)
  Not run: no test command resolved in this workspace, so nothing was executed to pass.
• New behavior is covered by a test (reproduce-first for bug fixes)

[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.
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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.