GeezerVison

why: Land the experimental ChainWorkspaceBuilder from #1054 on the new
builder package. It builds the window/pane tree via libtmux's chain API
(libtmux#685), which is unreleased and absent from published libtmux, so
the scaffold must import cleanly and fail with an actionable error rather
than crash on import.

what:
- Add src/tmuxp/workspace/builder/chain.py: ChainWorkspaceBuilder
  subclasses ClassicWorkspaceBuilder; guard the experimental import
  behind _HAVE_CHAIN so the module imports without the API
- build() raises exc.WorkspaceBuilderImportError when the chain API is
  absent; when present, port #1054's single-ForwardPlan build
- Register the `chain` entry point in tmuxp.workspace_builders
- Export ChainWorkspaceBuilder from the builder package
- Add tests: test_builder_chain.py importorskips the chain API; the new
  test_builder_chain_unavailable.py asserts selecting `chain` without the
  API resolves the class yet raises on build()

why: Tell users the chain builder exists, how to select it, and that it
is non-functional until libtmux's unreleased chain API ships.

what:
- Add docs/internals/api/workspace/builder/chain.md autodoc page with an
  unreleased-API warning; add it to the builder index grid and toctree
- Add an "Experimental chain builder" section to the custom-workspace-
  builders topic and list it under Reference
- Add a CHANGES entry under 1.74.0 noting the opt-in builder requires
  libtmux#685 and is non-functional until that ships

why: the chain builder's plan-building methods require the unreleased
libtmux._experimental.chain API (libtmux#685) and cannot execute on
released libtmux, so codecov flagged the unmeasurable lines as a
project-coverage drop.

what:
- Omit src/tmuxp/workspace/builder/chain.py from coverage until the
  experimental API ships and the builder becomes testable