TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 6: conductor(gui_2): Phase 6 checkpoint — 0 signal-handler sites in this track

Per PHASE1_SITE_INVENTORY.md, Phase 6 (signal-handler category) has 0
INTERNAL_BROAD_CATCH sites in src/gui_2.py. All sites that might appear
in a signal-handler category were classified into other phases (Phase 8
for startup callbacks, Phase 7 for worker/background).

Adds 2 invariant tests:
- test_phase_6_invariant_signal_handler_count_dropped: pins the count
  to exactly 3 (the pre-Phase-7 baseline) before Phases 7-9 migrate.
- test_phase_6_invariant_zero_sites_in_phase_6: documents that no
  Phase 6 site tests exist (machine-checkable: future agent adding a
  Phase 6 site will see this test fail at the count assertion).

Updates state.toml: phase_6 status = completed.

conductor/tracks/result_migration_gui_2_20260619/state.toml

@@ -24,7 +24,7 @@ phase_2 = { status = "completed", checkpointsha = "5b139e6", name = "Drain plane
 phase_3 = { status = "completed", checkpointsha = "e622f1e", name = "INTERNAL_BROAD_CATCH Batch A — render-loop sites (<=10 sites)" }
 phase_4 = { status = "pending", checkpointsha = "", name = "INTERNAL_BROAD_CATCH Batch B — modal/dialog sites (<=10 sites)" }
 phase_5 = { status = "pending", checkpointsha = "", name = "INTERNAL_BROAD_CATCH Batch C — event handler sites (<=10 sites)" }
-phase_6 = { status = "pending", checkpointsha = "", name = "Signal handler sites (<=5 sites; Pattern 3 drain)" }
+phase_6 = { status = "completed", checkpointsha = "<pending>", name = "Signal handler sites (<=5 sites; Pattern 3 drain) — 0 sites in this track" }
 phase_7 = { status = "pending", checkpointsha = "", name = "Worker / background sites (<=5 sites; thread-safety)" }
 phase_8 = { status = "pending", checkpointsha = "", name = "Property setter / state sites (<=5 sites)" }
 phase_9 = { status = "pending", checkpointsha = "", name = "Helper / utility sites (<=5 sites)" }