feat(provider): validate scope_filter against the v0.5 4-axis lattice

bifrost 0.7.0 (wire v0.5) makes agent_self canonical: the scope lattice is
now {end_user, group, tenant, agent_self}. Our store was MORE permissive than
bifrost's reference (no _validate_scope_filter), which silently 0-zeroed the
#295 cold recall instead of a loud 400. Now matched: search rejects an
out-of-lattice axis with InvalidFilter (-> memory.invalid_filter 400), agent_self
admitted. Purely additive — everything that validated before still validates.

Closes the parity gap our own foot-gun flag opened (bifrost-dev shipped the
lattice add #10 off it). Pin bumped bifrost>=0.6.1 -> >=0.7.0. Contract
search PRE-003 + lattice_axes test; 2 new store tests; full suite 429 green.

tests/test_provider_memory.py

@@ -10,7 +10,12 @@ from __future__ import annotations
 import types
 
 import pytest
-from bifrost.memory import IdempotencyConflict, InvalidArguments, RevisionMismatch
+from bifrost.memory import (
+    IdempotencyConflict,
+    InvalidArguments,
+    InvalidFilter,
+    RevisionMismatch,
+)
 
 from ratatoskr.provider.memory_store import (
     build_memory_provider_app,
@@ -221,6 +226,28 @@ async def test_search_non_dict_scope_filter_rejected():
         await store.search(_vec(1.0), top_k=5, scope_filter="u1")
 
 
+async def test_search_out_of_lattice_scope_axis_rejected():
+    # v0.5 scope lattice = {end_user, group, tenant, agent_self}; an axis outside
+    # it is InvalidFilter (-> memory.invalid_filter 400), matching bifrost's reference.
+    store = open_memory_store(":memory:", embedding_dim=EMBEDDING_DIM)
+    with pytest.raises(InvalidFilter):
+        await store.search(_vec(1.0), top_k=5, scope_filter={"bogus_axis": "x"})
+
+
+async def test_search_agent_self_axis_accepted():
+    # agent_self became canonical at wire v0.5 (#10) — admitted, not rejected.
+    store = open_memory_store(":memory:", embedding_dim=EMBEDDING_DIM)
+    await store.upsert_many(
+        [_chunk("a1", scope={"agent_self": "ratatoskr:smoke"})],
+        idempotency_key="k1",
+        ctx=_ctx(),
+    )
+    results = await store.search(
+        _vec(1.0), top_k=5, scope_filter={"agent_self": "ratatoskr:smoke"}
+    )
+    assert [r["chunk_id"] for r in results] == ["a1"]
+
+
 async def test_search_top_k_zero_returns_empty():
     # POST-001: at most top_k — zero means zero
     store = open_memory_store(":memory:", embedding_dim=EMBEDDING_DIM)