Require coordinated optional tmux operation guards

[tony]

Require libtmux's command policy (libtmux._internal.support.require_commands) before custom builder/plugin imports or shell code. A libtmux without it is refused. The package requires libtmux~=0.63.0 (see #1092); until that release exists the lock pins libtmux's feature head (5df9cb3e), whose version and control-identity probes are hook-free. Install a checkout with uv sync --locked.

Merge and release order: libtmux 0.63.0 is released first, from #779 and #780. This PR, stacked on #1092, requires libtmux~=0.63.0, which no installer can satisfy until then, so it merges only after that release, in the same change that drops the development [tool.uv.sources] pin. Nothing in this PR tags or releases.

Optional operations retain their individual requirements: stable tmux 3.7+ with a compatible client for control, 3.4+ for exact named-buffer operations, and 3.5+ for captured foreground shell output. Exact tmux 3.7 pane breaks require a window name. Malformed literal layouts refuse before dispatch on 3.3a, preserving running sessions; valid layouts, ordinary workspace operations, and portable percentage splits remain available.

Validation:

• Version check, optional-operation and workspace-layout cases pass locally after the move to libtmux._internal.support (29 passed, 12 skipped); earlier revisions of this branch passed 48 cases each on 3.3a, 3.4, 3.5, exact 3.7 and 3.7d. Installed-pair checks use the immutable dependency without PYTHONPATH.
• Dropping require_commands from the backend gate fails the three refusal cases (builder import, plugin import, shell code). The 3.3a layout test expects LibTmuxException on every release, because libtmux now raises InvalidLayout instead of a bare ValueError, so the release-dependent expectation is gone.
• Formatting, lint, mypy, the documented docs build and wheel/sdist builds pass. Docs retain the same 16 warnings. Replay verifies every final file against the independently reviewed source.

The earlier full suites (949 passed, 2 skipped on both 3.3a and 3.7d) belong to a prior candidate and remain separate corpus evidence. The core base (44da4788, run) has all eight hosted CI cells green; this restacked head (dc819005) is not claimed green.

Depends on the tmux 3.3a version check change.

Closes #1091.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.48%. Comparing base (44da478) to head (dc81900).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           bump-min-tmux-000    #1093      +/-   ##
=====================================================
+ Coverage              82.44%   82.48%   +0.03%     
=====================================================
  Files                     31       31              
  Lines                   2826     2826              
  Branches                 525      525              
=====================================================
+ Hits                    2330     2331       +1     
+ Misses                   357      356       -1     
  Partials                 139      139              

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