Pluggable terminal backend (1/2): widen TerminalBackend ABC + construction factory

[mrw1986]

Prep refactor for a pluggable terminal backend, per #276 (herdr backend + the Windows interest you raised). No behavior change — tmux stays the default and the only implementation.

This is PR 1 of 2. PR 2 adds HerdrBackend on top of this seam.

What changes

• Widen TerminalBackend (terminal_runtime/backend_types.py) from 5 methods to the full contract TmuxBackend actually exposes — 21 @abstractmethod + 3 safe no-op defaults (set_pane_style, refresh_pane_logs, save_crash_log) — so a second backend shares one typed interface. Signatures were copied verbatim from TmuxBackend; a conformance test asserts TmuxBackend.__abstractmethods__ is empty.
• Single construction point make_terminal_backend() (terminal_runtime/backend_selection.py); all 11 direct TmuxBackend(...) call sites now route through it (kwargs preserved).
• herdr selector branch (stub → NotImplementedError) added to the factory, TerminalBackendSelection.get_backend, and TerminalLayoutService.create_auto_layout. tmux paths are unchanged.

Why widen the ABC (the open question from #276)

The ABC declared only 5 methods, but callers rely on ~24 of TmuxBackend's. Widening to the real contract gives HerdrBackend (and a future Windows-native backend) one typed interface to implement, instead of duck-typing against TmuxBackend's de-facto surface. If you'd prefer a narrower ABC or a different split, I'm happy to adjust — this PR is meant to settle that shape before the herdr implementation lands.

Testing

• Seam suite green: 340 tests across test_terminal_runtime_backend_selection, test_tmux_backend, test_detect_terminal, test_terminal_runtime_layouts, test_v2_tmux_start_layout, test_ccbd_project_clear, the layout CLIs, the *_session_ensure_pane tests, etc. (the live-smoke ones need a real tmux).
• New tests: factory returns TmuxBackend for tmux/None and raises for herdr; selector + layout herdr stubs raise; TmuxBackend ↔ ABC conformance.

Scope

Touches only terminal_runtime/*, the 11 construction sites, and tests. No changes to the mailbox / lease / recovery / coordination layers — reply capture is session-file based, so those are backend-agnostic and unaffected.