Test coverage for TuiRuntime error paths

[rorygraves]

Background

TuiRuntime has several error paths (command coalescing overflow, unhandled exception → TermFlowErrorCmd → error overlay, Sub failure, shutdown mid-frame) that are structurally exercised but not explicitly tested.

Regressions here become user-facing terminal corruption, missed shutdowns, or silently swallowed errors.

Goal

Targeted tests for each runtime error path.

Scope

1. Test: unhandled exception in update surfaces via error overlay.
2. Test: Sub thrown exception doesn't kill the runtime.
3. Test: shutdown flushes pending Cmds gracefully.
4. Test: CmdBus backpressure / overflow under burst load.
5. Test: FCmd failing Future surfaces as TermFlowErrorCmd.

Acceptance

• Each listed path has a dedicated test.
• Coverage on TuiRuntime improves measurably.

Part of Cross-cutting — Semantic model & test debt.

[rorygraves]

Verification pass after the §4.7 coverage uplift: most of the scoped error paths are now exercised, but two real gaps remain.

Covered:

1. ✅ FCmd failing Future surfaces as TermFlowErrorCmd — TuiRuntimeSpec:121 ("FCmd failure path and surfaces fallback unexpected message").
2. ✅ Shutdown flushes / closes cleanly — TuiRuntimeSpec:102 ("runtime shutdown closes terminal backend") + :254 ("runtime restores cursor and exits alt buffer when render is interrupted").
3. ✅ Error overlay / banner formatting + truncation + frame immutability — SimpleANSIRendererSpec:17–55.
4. ✅ Sub cancel-throw resilience — SubSpec:137 + TuiRuntimeSpec:76 ("cancelAllSubscriptions continues when one cancel throws").

Still missing:

1. ❌ Sub mid-stream exception (callback in Sub.Every / Sub.InputKey throws while running) doesn't crash the runtime.
2. ❌ CmdBus backpressure / overflow under burst enqueue.

Scoping the issue down to those two — keeping it open as a focused follow-up. Not a 1.0 blocker.

[rorygraves]

Implemented in #214 (merged): dedicated tests for the TuiRuntime error paths (unhandled update exception → error overlay, Sub failure isolation, graceful shutdown flush, CmdBus overflow, FCmd failure → TermFlowErrorCmd).