Restore context-aware error footer (cursor-line error)

[monkopedia-coder]

Summary

Fixes #33 — a Compose-migration regression. Pre-migration, putting the cursor on a line with a compile error showed that error in the bottom footer; moving on/off lines updated it. That regressed: the footer only showed a generic "N message(s)" count and the editor never tracked cursor position. Error squiggles/gutter still worked — only the cursor→footer link was lost.

Fix

Wire the kodemirror session's onUpdate callback (fired on every dispatched transaction) to report the primary cursor's 1-based line to the view model. (The updateListener facet looked like the natural hook but is never dispatched in kodemirror 0.3.2, so onUpdate is the working path.) The view model maps that line onto the current diagnostics, and the footer shows the matching error (falling back to the count when the cursor isn't on an error line). Lint squiggles/gutter unchanged.

Tests (TDD — reproduced the regression first)

New ErrorFooterTest (e2e) asserts the three behaviors from #33:

1. diagnostics land on the right lines,
2. cursor on an error line shows its error in the footer,
3. moving the cursor updates/clears the footer.

Before the fix, (2) and (3) FAILED (Playwright timeouts), while (1) passed — i.e. the diagnostic line-offset was already correct; only the cursor→footer link regressed:

ErrorFooterTest > testCursorOnErrorLineShowsItsError FAILED
    com.microsoft.playwright.TimeoutError at ErrorFooterTest.kt:146
ErrorFooterTest > testMovingCursorUpdatesFooter FAILED
    com.microsoft.playwright.TimeoutError at ErrorFooterTest.kt:146
3 tests completed, 2 failed

Required new TestBridge hooks (error lines, cursor line, footer text + a deterministic cursor-mover action), since the bridge exposed none of that.

Validation

• Full clean :e2e:test green incl. ErrorFooterTest; existing suite stays green. :frontend:spotlessKotlinCheck clean.

Review note

@Monkopedia — touches editor UX (the restored footer behavior); please eyeball the final footer. Relates to #32 (broader error-surfacing UX).

Fixes #33

[monkopedia-reviewer]

Tier-2 signoff by Monkopedia (eyeballed live on adolin — restored cursor-line error footer, diagnostic lines confirmed correct). Reviewed to merge standard: correct cursor→line→diagnostic mapping, meaningful e2e ErrorFooterTest, CI green.