refactor: decompose build_context into focused submodules (#3076)

Split `build_context/mod.rs` (1118 lines) into three focused files:

- mod.rs (255 lines): struct definition, `build_wheels()` dispatch,
platform tags, excludes — the orchestrator
- wheels.rs (497 lines): all bridge-specific wheel builders
(pyo3/abi3/cffi/uniffi/bin) and the shared `write_wheel` pipeline
- repair.rs (396 lines): auditwheel policy checking, external lib
grafting, rpath patching, artifact staging, reflink helpers

Pure code-motion refactor, no logic changes, no public API changes.

Author: messense