Conflict detection: classify local and remote changes

[SashankBhamidi]

src/calendaring_sync/conflict.py:

classify(key, local_content, remote_etag, remote_content) -> ChangeClass

resolve(key, change_class, local_content, remote_content, resolver_fn)

ChangeClass enum: REMOTE_ONLY, LOCAL_ONLY, CONFLICT, DELETED_REMOTE, DELETED_LOCAL, UNCHANGED

Four cases:

1. Remote changed, local unchanged: server ETag differs from cached, local content matches cache. Safe to apply remote.
2. Local changed, remote unchanged: local content differs from cache, server ETag matches cached. Safe to push local.
3. Both changed: server ETag differs and local content differs from cache. Conflict. Surface to caller.
4. Deleted on one side: one side has the object, the other doesn't. Surface to caller.

resolver_fn receives both versions and returns the content to keep, or raises ConflictError to abort. The library never merges or makes opinionated decisions. That's the caller's job.

Tests: one unit test per ChangeClass case. resolver_fn is called for CONFLICT, not called for REMOTE_ONLY or LOCAL_ONLY. ConflictError propagates cleanly.