Engine Decoupling - Phase 3.3: Validation and usability hardening

[JohnLudlow]

Summary: Add deterministic validation coverage, terminal compatibility review, fallback-mode validation, and contributor/host documentation for the TUI adapter.

Parent: #3
Epic: #2

Objective: Ensure the TUI adapter is dependable across real environments and supported by operational guidance.

Acceptance Criteria:

• Deterministic validation or normalized snapshot coverage exists for key TUI flows.
• Monochrome and restricted-terminal fallback behaviour is defined and validated.
• Terminal capability caveats and known limitations are documented.
• Contributors can run and troubleshoot the TUI adapter using repository guidance.

Plan doc:

• docs/plans/engine-decoupling-epic/phase-3-tui-adapter-development/phase-3-3-validation-and-usability-hardening.md