refactor: shared build runner across platforms

[henryiii]

Alternative to #2903, this time using Claude Fable 5 instead of Claude Opus 4.8. This is not just a one to one; this one doesn't have nearly as much restraining it, and I didn't tell it where I wanted to go later (--stage). But otherwise it's also an unedited output of a plan.

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What

Extracts the per-wheel build loop that was duplicated across all six platform modules into a shared runner (cibuildwheel/platforms/runner.py), driven by a per-platform builder class with clearly-named steps:

setup → before_build → build_wheel → repair_wheel → audit_wheel
      → test_wheel → move_to_output → cleanup

• Builder(ABC, Generic[PathT]) — one method per step. Generic over the wheel-path type because Linux wheels live in the container as PurePosixPath; everywhere else they're host Paths.
• run_builds() — a single flat loop that owns everything that was previously copy-pasted six times: log.build_start/build_end/step accounting, compatible-wheel (abi3) reuse, the none-any.whl check, exactly-one-repaired-wheel validation, AlreadyBuiltWheelError, the test gate, output bookkeeping, and cleanup. The loop body reads top-to-bottom like the old per-platform loops — there's no step registry or hook indirection.
• HostBuilder — the step bodies shared by the five non-Linux platforms (before_build, default repair, audit, move-to-output, cleanup).
• run_before_all() / fatal_on_called_process_error() — replace the per-platform copies of the before_all hook and the CalledProcessError → FatalError wrapper (Linux keeps its per-container scope and troubleshoot() hook).

Each platform's build() shrinks to ~15 lines; platform-specific structure stays where it was and stays visible: macOS keeps its per-arch test loop (universal2/Rosetta), iOS its testbed flow, Android its dual build/android environments (the previous BuildState pattern was the prototype for this design), pyodide its pyodide venv testing, and Linux keeps get_build_steps() grouping and the OCIContainer lifecycle, invoking the runner once per container. build_in_container() keeps its signature (it's mocked in option_prepare_test.py), and the PlatformModule interface is unchanged.

Behavior changes

The duplication had let the platforms drift; this unifies the unjustified differences (one commit per platform, each itemized in its commit message):

Bugs fixed

• pyodide: compatible-wheel reuse hit a latent NameError (assigned built_wheel, read repaired_wheel) — unreachable today only because pyodide wheels are never reused
• pyodide: a repair command producing no wheel raised a raw StopIteration instead of RepairStepProducedNoWheelError
• Linux: dependency-versions with packages: crashed because the host-side per-identifier temp dir was never created
• Android: test-environment / CIBW_TEST_ENVIRONMENT_ANDROID was documented but silently ignored; it's now applied to before-test, the wheel install, and the testbed invocation

Unified behavior

• repair must produce exactly one wheel on every platform (macOS/Windows/iOS gain RepairStepProducedMultipleWheelsError, previously Linux/Android-only)
• Android gains the AlreadyBuiltWheelError duplicate-name check; its redundant post-repair none-any.whl check is dropped (the built wheel is still checked, like the other platforms)
• Windows: the "ARM64 wheels cannot be tested" warning now only appears when a test command is actually configured
• iOS: a test-suite failure raises FatalError instead of calling sys.exit(1) (same exit code)
• pyodide: before_test no longer receives an undocumented {wheel} placeholder; test_command now receives {wheel} like Linux/macOS/Windows
• AssertionError("uv not found") → FatalError (macOS, Android)
• cosmetics: one compatible-wheel reuse message everywhere, uniform log.step_end() accounting, host temp dirs removed with ignore_errors=True (pyodide previously never removed them), Android gains the "was moved to … instead of" warning

Testing

• unit_test/runner_test.py (new): a recording FakeBuilder covers step ordering, compatible-wheel reuse (build/repair/audit/move skipped, test still runs), 0/2-wheel repair errors, duplicate-name and none-any errors, test gating, and the error wrapper + troubleshoot hook.
• Existing suites pass unchanged: 802 unit tests (including the build() monkeypatch seams and option_prepare_test.py), doctests, ruff, strict mypy (3.11 + 3.14), pylint.
• Integration run locally: pyodide (4 passed, end-to-end builds) and Linux in Docker — test_troubleshooting + test_before_all (11 passed) and test_custom_repair_wheel + test_abi_variants (6 passed, covering repair validation and compatible-wheel reuse). macOS CPython/Windows/iOS/Android rely on this PR's CI.

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