Let the server decide whose memory a caller may read

A holder is now somebody who can prove it. Schema v11 stores a sha256 of a
per-holder token beside the directory row, so a copied database cannot replay
it and the plaintext is shown exactly once. The operator token mints and
revokes those tokens; a holder token reads that holder's own memories and
nothing else; naming somebody else without the operator token is refused with
both ids in the message.

The visibility rule was leaking before this: browse had none at all, and the
search rules short-circuited to the whole library whenever no holder was
named. The rule is now written out in five copies with one parameter carried
four times, and a test compares the copies so they cannot drift — semgrep
rejects an interpolated constant, so the copies are not a choice.

Eyes carry three states on purpose. NULL means the whole library, which is
what a process that owns the store is entitled to; a holder id means that
holder; the empty string means nobody, which is what a tokenless HTTP caller
is. Making NULL mean nobody instead would have forced every internal caller —
CLI, proofs, the compiler, the bench harnesses — to name eyes, contradicting
the rule that a local process reads as its own trusted principal.

The capability card answers "can this store answer my kind of question" before
anything is sent. It is derived from what the store holds, so it cannot
promise a modality the store lacks, and it carries no memory text, so it is
safe to read before trusting the store. HTTP, CLI and one MCP tool all serve
it without a token.

backend/src/ely_eye/memory.py

@@ -17,6 +17,7 @@ from . import lifecycle_store, trust
 from .cache_fabric import CacheFabric
 from .config import Settings, get_settings
 from .db import Database
+from .holder_store import HolderStore
 from .schemas import (
     ArchiveReport,
     ConflictRecord,
@@ -75,7 +76,8 @@ class MemoryService:
         """Store one statement as a memory owned by a holder.
 
         Repeating the same sentence as the same holder updates the memory instead of
-        duplicating it; two holders holding the same sentence keep two memories.
+        duplicating it; two holders holding the same sentence keep two memories. A holder nobody
+        registered is added to the directory here, so a referral to it has something to point at.
         """
         statement = text.strip()
         if not statement:
@@ -89,6 +91,7 @@ class MemoryService:
             )
 
         moment = utc_now()
+        HolderStore(self.db).register_minimal(holder_id)
         digest = hashlib.sha256(f"{holder_id}\n{statement}".encode()).hexdigest()
         source_id = f"mem-{digest[:16]}"
         self.db.upsert_sources(