What

Review all 135 public API headers for stability, add [[deprecated]] markers to legacy types, and remove any dead/unused APIs before the v1.0 freeze.

Part of #670

Why

• v1.0 represents an API stability commitment — breaking changes require major version bump after freeze
• Queue consolidation reduced public types from 8 to 2 but legacy types may still exist without deprecation markers
• Migration guide references v3.0.0 changes but no [[deprecated]] markers exist in current code
• Users need clear signals about which APIs are stable vs. transitional

Where

• include/kcenon/thread/ — all public headers (135 files)
• docs/advanced/MIGRATION.md — migration guide referencing v3.0.0 changes
• Legacy types mentioned: kcenon::thread::result<T>, kcenon::thread::result_void, kcenon::thread::error

How

Technical Approach

1. Inventory all public types, functions, and classes across headers
2. Cross-reference with MIGRATION.md to identify legacy/transitional APIs
3. Add [[deprecated("Use X instead. Will be removed in v2.0")]] to legacy types
4. Remove any truly dead code (unused after queue consolidation)
5. Document frozen API surface in a concise reference

Acceptance Criteria

• All public headers reviewed for API stability
• Legacy types marked with [[deprecated]] and migration message
• No dead/unreachable code in public headers
• Migration guide updated to reflect v1.0 API surface
• Build passes with deprecation warnings (not errors)