Make a cartridge a scope you mount, not only a bundle you export

A cartridge used to be an export: materialize it, hash it, and never let it
change behaviour again. The transactive-memory promise needs the opposite —
mounting the home cartridge must make the work cartridge's memories
unfindable, and appending to a cartridge must make a new memory findable
through it.

Cartridge scope (schema v2 -> v3):
- cartridge_atoms (which atoms are in which cartridge) and cartridge_mounts
  (which cartridges this machine has mounted now); both rebuilt on reopen,
  because a mount is a machine-local state, not a fact about the data.
- mount / switch / unmount / append refuse a cartridge that was built for a
  different runtime, naming every differing field instead of a boolean.
- append rewrites atoms.parquet and sparse_index.bm25.zst, recomputes DNA and
  the KV fingerprint, and keeps the cartridges row in one SQLite transaction;
  the files are written first so a failed export cannot leave a half-cartridge
  in the database.
- RetrievalService.search() now ranks inside the mounted scope by default, and
  an explicit cartridge_ids filter overrides the mount. Nothing mounted means
  the whole library, so an empty store behaves as it did before.
- forget() removes the forgotten atoms from every cartridge's membership:
  forgetting must not leave a memory retrievable through a cartridge.

KV fingerprint (kv_fingerprint.py, stored in the manifest):
- model / tokenizer / chat-template hashes, the RoPE block (factor,
  original_max_position_embeddings, mrope_interleaved, mrope_section,
  partial_rotary_factor), the adapter hash, the context length, the engine
  version, and the fingerprint format version.
- A snapshot we cannot read is recorded as unverifiable and listed in
  unverifiable_fields; a blind machine refuses to mount rather than pretending
  the runtime matches, because a wrong RoPE factor silently corrupts retrieval.

Two defects found by running the new tests, not by reading:
- atoms.parquet could not export a memory at all: pyarrow refuses a struct with
  no child field, so metadata={} failed the whole table. The six JSON columns
  are now written as JSON text and decoded by the cartridge reader.
- materialize() inserted cartridge membership before the cartridges row existed,
  so the foreign key rejected every cartridge.

api.py crossed 500 lines, so the cartridge and mount routes moved to
api_cartridges.py. That module must not use `from __future__ import
annotations`: FastAPI then resolves Depends(settings_dep) against module
globals, where the injected dependency is only a parameter of register(), and
every route starts demanding settings as a query parameter.

Verification: 138 pytest passed (26 new: 10 fingerprint, 16 scope), Go
build/vet/gofmt/test clean, and scripts/mcp_write_smoke.sh now proves the whole
loop across two processes - mount narrows search, a memory written after the
upload is invisible until append_to_cartridge, unmount restores the library,
and rewriting the RoPE factor makes mount_cartridge refuse with "rope: factor:
cartridge 4.0 vs local 2.0". ruff clean on every touched file (the 20 remaining
package errors are in files this change did not touch; HEAD had 25). mypy: one
error, the pre-existing np.savez stub artifact at cartridge_assets.py:329
(HEAD:320). Repo pyright: 62 errors, unchanged from the p0-3 ledger.

backend/src/ely_eye/db.py

@@ -187,6 +187,7 @@ class Database:
         since: datetime | None = None,
         until: datetime | None = None,
         min_trust: float | None = None,
+        cartridge_ids: list[str] | None = None,
     ) -> FtsPage:
         """BM25 over the FTS index joined to memories that are open right now.
 
@@ -215,6 +216,11 @@ class Database:
                             json_extract(a.trust_json, '$.model')
                         ) >= ?
                   ))
+                  AND (? IS NULL OR EXISTS (
+                        SELECT 1 FROM cartridge_atoms AS ca
+                        WHERE ca.atom_id = f.atom_id
+                          AND ca.cartridge_id IN (SELECT value FROM json_each(?))
+                  ))
                 ORDER BY score
                 LIMIT ? OFFSET ?
                 """,
@@ -234,6 +240,8 @@ class Database:
                     None if until is None else until.isoformat(),
                     min_trust,
                     min_trust,
+                    None if cartridge_ids is None else to_json(cartridge_ids),
+                    None if cartridge_ids is None else to_json(cartridge_ids),
                     limit + 1,
                     offset,
                 ),
@@ -330,6 +338,17 @@ class Database:
             ).fetchall()
         return [row_to_source(row) for row in rows]
 
+    def get_sources(self, source_ids: list[str]) -> list[SourceRecord]:
+        if not source_ids:
+            return []
+        with self.connect() as connection:
+            rows = connection.execute(
+                "SELECT source_id, path, kind, mime, sha256, size_bytes, created_at, metadata_json "
+                "FROM sources WHERE source_id IN (SELECT value FROM json_each(?))",
+                (to_json(source_ids),),
+            ).fetchall()
+        return [row_to_source(row) for row in rows]
+
     def cartridge_count(self) -> int:
         with self.connect() as connection:
             return int(connection.execute("SELECT COUNT(*) FROM cartridges").fetchone()[0])