Gate unsafe optional tmux features independently

[tony]

Keep ordinary MCP tools on tmux 3.3a while refusing unsafe optional operations before their effects. This is stacked on the core minimum change.

Operation	Required tmux
Input, pane capture, polling, completion channels, run_command, show_buffer and direct paste_buffer	3.3a
load_buffer (stages a named buffer) and delete_buffer (exact named deletion)	3.4, refused before dispatch; a 3.3a daemon creates no buffer
Nonempty paste_text (its temporary buffer needs exact named cleanup on failure)	3.4, refused before the buffer is staged
Literal select-layout on 3.3a	Unknown names and malformed serialized headers refused before dispatch; valid layouts unchanged

load_buffer joined the 3.4 group after review: it staged a named buffer that a 3.3a daemon could never delete, and tmux limits only automatic buffers, so repeated calls grew the daemon. Owned subprocesses bypass the parent library's methods, so the dispatcher forwards each literal argv to libtmux's command policy (libtmux._internal.support.require_commands) and carries an optional minimum through the existing process budget. Startup and every tool refuse a libtmux without that function. Empty paste_text has no input effect on any release and returns before touching tmux.

Not applicable here, because the server has no such code: library-owned control acquisition (tools poll; nothing attaches a control client), captured run-shell stdout (run_command types into a pane), break-pane, a raw split-window -p, and a private control socket alias. Session and window renames on exact 3.7 reach tmux, which rejects : and . in names with no effect; a native probe confirmed both refusals leave the names unchanged. Per-feature support states are in the compatibility reference; none raises the server's minimum.

The package requires libtmux>=0.63.0 (see #139), and the development lock pins the implementation from Python feature PR780 (5df9cb3e) until that release exists.

Merge and release order: libtmux 0.63.0 is released first, from #779 and #780. This PR, stacked on #139, 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.

Validation:

• The feature, buffer and version check tests pass on the installed tmux after the restack onto the renamed core (41 passed, 10 skipped, the native 3.2a and unpatched 3.3 refusal lanes not configured). Ruff format/lint, mypy and uv lock --check pass.
• Hosted CI is green on the core base a2b4b3af (run); the restacked head ba573fee is not claimed green.
• Deliberate breaks, each on tmux 3.3a. Removing the forwarded literal command policy from the dispatcher fails the named-buffer identity, delete-tool and layout cases (six failures among them). Removing the 3.4 minimum from paste_text staging fails test_managed_paste_checks_cleanup_before_staging. Removing the 3.4 minimum from load_buffer fails test_load_buffer_stages_only_where_it_can_be_deleted with DID NOT RAISE, which asserts that only the user's buffer remains. The unmodified tree passes.
• The hook-free version probe inherited from the core change is covered by its own native test, which fails when a display-message call is put back into the probe.

Support is qualified on Linux/WSL by local native runs and the hosted Linux matrix; macOS and Windows are not claimed. Opaque tmux or shell programs and configured alias bodies keep native semantics, and library-owned metadata and guard commands assume native built-in semantics.

Closes #138.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.73%. Comparing base (a2b4b3a) to head (ba573fe).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           bump-min-tmux-000     #140      +/-   ##
=====================================================
+ Coverage              86.52%   86.73%   +0.21%     
=====================================================
  Files                     46       46              
  Lines                   4053     4057       +4     
  Branches                 606      607       +1     
=====================================================
+ Hits                    3507     3519      +12     
+ Misses                   381      374       -7     
+ Partials                 165      164       -1     

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