feat(cache-manager): add add() for atomic store-if-absent of application values

[allen0099]

Closes #65.

Change

CacheManager.add(key, value, ttl=None) -> bool stores a JSON value only when the key is free and returns whether it did.

• It uses the same key prefix, JSON encoding and default_ttl handling as set(). A non-JSON-serializable value raises TypeError.
• It calls backend.set_if_absent (Add an atomic set-if-absent (SET NX EX) to BaseCacheBackend for lock / slot acquisition #62), so the check and the write are one atomic operation on Memory, Redis and Memcached. Of several concurrent callers, exactly one gets True.
• An expired key counts as free. A key that holds an undecodable value counts as present, so add() returns False for it, even though get() and get_or_set() treat it as a miss. The docstring and the guide both say so.
• set() and add() now share a private _encode() helper that builds the JSON CacheEntry.

Docs:

• docs/APP_CACHE.md has an example and a behavior bullet, and the Memcached note now lists add().
• The README features line mentions add().
• CLAUDE.md is updated.
• [Unreleased] in CHANGELOG.md has an Added entry.

Tests

The new tests in tests/test_cache_manager.py run against Memory, and against Redis when a test server is configured:

• The value is stored when the key is free.
• An existing value is never overwritten.
• 20 concurrent add() calls: exactly one succeeds, and the stored value is the winner's.
• An explicit ttl applies, and the key can be added again after it lapses.
• default_ttl is used when no ttl is given.
• A corrupted entry counts as present.
• TypeError is raised and the key is left free.

Local results:

• Full suite against the throwaway Redis (6380) and Memcached (11212) test servers: 689 passed, 1 skipped. Coverage is 100%, including manager.py.
• ruff, mypy --strict and zensical build --strict are clean.