feat: add Command Timeline Phase 1 checkpoint

[HelloThisWorld]

Summary

Add Command Timeline Phase 1 as the v1.2.1 engineering checkpoint. This introduces a pane-owned, bounded in-memory command index backed by native shell-integration marks, plus lifecycle/result tracking and stable native mark identity across reflow and pruning.

Phase 1 intentionally exposes only C++ model seams; timeline UI, navigation, and public release packaging remain out of scope for this pull request.

Related issues

None.

Detailed changes

• Add the Command Timeline model, stable pane-scoped IDs, bounded command text, capability classification, trusted exit-code results, cache invalidation, and pane view state.
• Connect OSC 133 shell-integration lifecycle updates to the pane-owned index without output heuristics or terminal-output storage.
• Preserve transient native mark identity through reflow, circular eviction, scrollback clearing, and pane close.
• Add focused model and native OSC integration tests covering repeated identical commands, incomplete/out-of-order lifecycle events, result mapping, bootstrap behavior, invalidation, reflow, and cleanup.
• Advance engineering checkpoint metadata to v1.2.1 while keeping public v1.2.0 release links and release state unchanged.
• Guard v1.2.1-v1.2.4 checkpoint tags so they run validation only and cannot enter full packaging/publishing jobs.

Compatibility and migration: no persisted schema changes are introduced. Timeline data is pane-local and in-memory, and existing workspace fallback behavior remains compatible.

Validation performed

All checks ran on Windows x64 Debug with the repository's Microsoft Terminal toolchain:

• msbuild.exe src\cascadia\UnitTests_Control\Control.UnitTests.vcxproj /p:Configuration=Debug /p:Platform=x64 /p:WindowsTerminalBranding=WinTerm "/p:SolutionDir=<repo>\\" /verbosity:minimal /m:1 — passed (only existing APPX4001 warnings).
• .\bin\x64\Debug\UnitTests_Control\TE.exe .\bin\x64\Debug\UnitTests_Control\Control.Unit.Tests.dll "/select:@Name='ControlUnitTests::CommandTimelineTests::*'" /logOutput:Low — 12/12 passed.
• .\scripts\winterm\test.ps1 -Suite Smoke -Configuration Debug -Platform x64 — passed.
• .\scripts\winterm\test-release-workflow.ps1 — passed.
• .\scripts\winterm\verify-version.ps1 -RequireTag — passed.
• PowerShell parser checks for modified scripts, JSON/XML parsing, PyYAML workflow parsing, and CRLF-aware git diff --check — passed.
• The v1.2.1 tag-triggered checkpoint workflow completed successfully; full validation/build/release jobs were skipped by the checkpoint guard as designed.

Checklist

• The change is focused and does not include unrelated formatting.
• Tests were added or updated where appropriate.
• All tests claimed above actually ran and passed.
• User-facing behavior and limitations are documented in this repository.
• Version or schema changes include compatibility and migration notes.
• Package identity, winterm.exe, and Microsoft Terminal coexistence remain isolated.
• No command text, terminal output, clipboard content, credentials, or private paths are logged.
• New source and script files contain the appropriate MIT license header.
• I did not include generated build output, secrets, certificates, or local absolute paths.