refactor(cache): split cache.py into smaller modules (#422)

[allen0099]

Closes #422.

fastapi_cachex/cache.py had grown to about 1,800 lines. This PR splits it into private modules next to it. The public API and behaviour do not change.

Module	Contents
_key_builders.py	build_cache_key, default_key_builder, key builder checks
_vary.py	vary= validation and the key components it adds
_cache_control.py	CacheControl, the decorator's Cache-Control, checks for responses that must not be shared
_stored_response.py	cacheable headers, Age, ETags, 304s, the stored CacheEntry
_rendering.py	running the handler, building its response, dependency headers (#233)
_callables.py	whether a handler or key builder returns a coroutine
cache.py	the decorator, invalidate(), the credential-bypass warnings (about 970 lines)

How

• Every top-level definition was moved verbatim, together with its leading comments, by name.
• A review compared the source of every moved definition on master and on this branch with ast.get_source_segment. All of them are byte-identical, with one exception: the decorator now calls the new _entry_for() instead of building the CacheEntry inline, so the clock behind stored_at and Age lives in one module.
• Module-level state (_now, _ADAPTER_ATTR, the _BypassWarner lock) exists in exactly one place.
• There are no import cycles.

Compatibility

• Every name fastapi_cachex.cache defined on master is still importable from it. These names are now listed in __all__, and tests/test_cache_module.py pins them.
• Every module that logs uses the documented fastapi_cachex.cache logger. The new test checks this too.
• The docs/api references (fastapi_cachex.cache.cache, invalidate, default_key_builder) still resolve.
• Names cache.py only imported for its own use are no longer reachable through fastapi_cachex.cache. For example, from fastapi_cachex.cache import CacheEntry now fails; import it from fastapi_cachex.types. Their documented homes are unchanged.
• Private names moved, so code that monkeypatches fastapi_cachex.cache._now must patch fastapi_cachex._stored_response._now. The repository's own tests are repointed.
• There is no changelog fragment, because behaviour does not change (changelog.d/README.md).

Docs

CACHE_FLOW.md (EN and zh-TW) no longer says that all the behaviour lives in cache.py. It now names the module for each part.

Checks

• pre-commit
• mypy fastapi_cachex --strict, mypy tests and mypy scripts
• the full suite with live Redis and Memcached: 1829 passed, coverage 99.85%, each new module 100%
• both zensical builds with --strict
• an independent read-only review: no blockers. Two of its nits are fixed: the helper _key_builders imported from _rendering moved to _callables.py, and the __all__ comment was reworded.