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/retrieval.py

@@ -11,6 +11,7 @@ from __future__ import annotations
 import base64
 from collections import defaultdict
 
+from . import cartridge_store
 from .config import Settings, get_settings
 from .db import Database
 from .embeddings import EmbeddingService
@@ -44,11 +45,16 @@ class RetrievalService:
         if limit <= 0:
             raise ValueError(f"search needs a positive limit, got {limit}")
         offset = decode_cursor(cursor)
+        criteria = (filters or SearchFilters()).model_dump(exclude_none=True)
+        scope = criteria.get("cartridge_ids")
+        if scope is None:
+            scope = cartridge_store.mounted_ids(self.db) or None
+        criteria["cartridge_ids"] = scope
         page = self.db.search_atoms(
             query,
             limit=limit * CANDIDATE_WINDOW,
             offset=offset,
-            **(filters or SearchFilters()).model_dump(exclude_none=True),
+            **criteria,
         )
         rankings: list[list[str]] = [[atom_id for atom_id, _ in page.rows]]
         scores: dict[str, dict[str, float]] = defaultdict(dict)
@@ -56,6 +62,9 @@ class RetrievalService:
             scores[atom_id]["sparse_score"] = bm25
         if self.embeddings.enabled():
             dense = self._dense_search(query, limit * CANDIDATE_WINDOW)
+            if scope:
+                allowed = cartridge_store.atoms_of_cartridges(self.db, scope)
+                dense = {atom_id: score for atom_id, score in dense.items() if atom_id in allowed}
             rankings.append(list(dense))
             for atom_id, cosine in dense.items():
                 scores[atom_id]["dense_score"] = cosine