fix: keep writes inside the store's scope; refuse operator keys on writes

[thorwhalen]

Closes #14

What changes

• Scope contradictions raise. _build_doc used to merge as dict(filter, **k, **v), so a key or value repeating a scope field with a different value wrote the doc outside the store's scope. It now raises ValueError naming the field, comparing as MongoDB does (BSON equality: True is not 1, subdocument field order matters). $eq/$in scope fields are checked by membership; other operator scopes ($ne, $gt, ...) are not checked and such writes are let through (documented). Dotted scope fields refuse writes with a clear error (those writes produced literal dotted field names before). Writes that agree with the scope produce exactly the same documents as before.
• No operators in write/delete keys. __setitem__ (replace_one), __delitem__ (delete_one), MongoCollectionMultipleDocsPersister.__setitem__ (its delete_many) and MongoBulkWritesMixin now go through _write_filter_for_key, which refuses keys containing $-prefixed field names or regex values (re.Pattern, bson.regex.Regex) at any depth. The check runs before any call to the collection; MongoCollectionMultipleDocsPersister also builds and validates every new doc before its delete_many, so a refused write deletes nothing. Keyword-only allow_operators_in_write_keys=True (constructor arg, or class attribute) restores the old behaviour.
• Replace/delete queries confined by on_write_filter too (previously only by filter), so a store stamping on_write_filter={"tenant": "a"} over an unscoped filter can no longer replace or delete another tenant's docs.
• Reads are unchanged: s[k] / k in s keep accepting query keys (used by recipes.disallow_sourced_interval_overlaps), and remain confined by the scope through $and.
• Tests: mongodol/tests/write_scope_test.py, using a recording stand-in for the collection (no server needed; hosted CI also runs the full suite against its MongoDB service).

Compatibility

Behaviour changes for: writes that contradicted the scope; write/delete keys with operators or regexes; writes through dotted scope fields; and stores whose on_write_filter stamps a field that older docs lack — replace/delete by key no longer matches those older docs (an upsert on an existing _id then fails with a duplicate-key error, and MultipleDocs writes leave the old docs in place). mongodol has no fleet dependents.

Review

Independent refute-review by a sub-agent found three blockers in the first version (regex keys still wiped the scope; MultipleDocs deleted before validating; operator scopes broke legitimate writes) plus several should-fixes. All were fixed in c16d08f; a re-review found no remaining blockers. Remaining known gaps: operator scopes other than $eq/$in fail open (documented); no test yet for the bulk-write mixin path.

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