GH-108: Fix control panel status label stuck at Initializing

[ehkropf]

Summary

• Replace hardcoded "Initializing..." label in control panel with dynamic m_computationPhase that shows the latest status message during computation setup
• Show iteration/residual display once Newton iterations start (unchanged behavior)
• Display convergence/failure summary after computation completes (previously showed nothing)

Fixes #108

Test plan

• Build succeeds
• All 314 tests pass, no regressions
• Manual: Load thesis example, click Compute — verify label shows "Computing conformal map..." initially
• Manual: Verify label switches to "Iteration: N / Residual: X" once Newton starts
• Manual: Verify label shows "Converged (N iterations)" on completion

🤖 Generated with Claude Code

[ehkropf]

Blind Review Findings

Ran parallel review agents (code-reviewer, silent-failure-hunter, code-simplifier). Summary:

Fixed in 569001c

1. Null guard on mp_controller in onComputationComplete() — Every other method in MainWindow guards this pointer; the new callback did not. Added if (!mp_controller) return; for consistency.

2. Stale label on first frame — Brief window between m_isComputing becoming true and the first status message arriving via the message queue. During this window, m_computationPhase showed whatever it had before (empty or previous completion text). Now set to "Computing..." when computeMapping() succeeds.

3. Cancellation displayed as failure — User-initiated cancel produced "Failed: Computation cancelled". Added wasCancelled() accessor to GuiController (reads m_cancelRequested atomic, which remains true until next computeMapping() resets it). onComputationComplete() now checks cancellation first and shows "Cancelled" instead.

Confirmed non-issues

• Thread safety — All reads/writes to m_computationPhase happen on GUI thread (status updates delivered via mutex-protected message queue, completion fires from update()). Safe.
• Stale state on reload — clear() and loadMap() both fire onStatusUpdate() which updates m_computationPhase. Handled by existing callback chain.
• Exception safety — Controller accessors are simple inline getters returning member values. Cannot throw.