Make corroboration depend on the claim, not on the size of the library

An independent reviewer broke the quarantine rule twice by running it. In a
two-atom library, a second source repeating the claim word for word could not
vouch for it; 40 unrelated memories could. The old rule handed the choice of
words to rare_terms, which drops words common to the corpus, so the corpus size
and the attack's own words decided whether a stranger was believed.

Corroboration now demands the claim's three longest words, with the instruction
phrases stripped out, ANDed by the new text_index.fts_all_expression. Neither
the library's size nor the attack can flip it, and both cases are tests.

Also from that review:
- A question is a read. The DF cache no longer remembers a word the corpus has
  never used, so an unauthenticated search cannot grow the store one row per
  novel word. "A question is not a write" was false for /api/search and /api/ask.
- The prompt now states that packed evidence is quoted material, not an
  instruction: once a quarantined sentence is corroborated it does reach the model.
- ForgetReport's semantic_cache_cleared reached the frontend as well as Go.
- runtime.py: process_vision_info returns a third element only when
  return_video_kwargs is set, and this call does not set it, so the unpack takes
  the first two. Two pre-existing pyright errors go, behaviour is unchanged.

CHECKLIST corrected against measurement: 13 renamed lines rather than 11, and
10 English / 9 Chinese injection signals rather than 11 / 8.

Gates: pytest 312 passed, ruff clean, mypy 24 errors in 9 files (net zero),
pyright 0 errors on the five touched files, npm build and eslint clean,
scripts/mcp_write_smoke.sh PASS.

backend/tests/test_text_index.py

@@ -18,6 +18,7 @@ from ely_eye.schemas import MemoryOrigin
 from ely_eye.store_search import search_atoms
 from ely_eye.text_index import (
     RAREST_KEEP,
+    fts_all_expression,
     fts_expression,
     fts_query,
     fts_terms,
@@ -179,6 +180,29 @@ def test_a_query_of_one_word_is_never_filtered() -> None:
     assert rare_terms(connection, ["the"]) == ["the"], "dropping the only word would leave nothing to match"
 
 
+def test_a_word_the_library_lacks_leaves_nothing_behind() -> None:
+    """A question is a read: a stranger's vocabulary must not grow the store one row per novel word."""
+    connection = _library(_shelf("pdf"))
+
+    rare_terms(connection, fts_terms("quixotic pdf"))
+
+    cached = connection.execute("SELECT term FROM term_df").fetchall()
+    assert [row[0] for row in cached] == [fts_expression(["pdf"])], (
+        "only a word the library actually holds is worth remembering"
+    )
+
+
+def test_an_all_expression_demands_every_word_it_is_given() -> None:
+    connection = _library(["the archive thermostat holds eighteen degrees", "the archive is closed"])
+
+    hits = connection.execute(
+        "SELECT atom_id FROM atom_fts WHERE atom_fts MATCH ?",
+        (fts_all_expression(["archive", "thermostat"]),),
+    ).fetchall()
+
+    assert [row[0] for row in hits] == ["a0"], "one shared word must not match a whole sentence"
+
+
 def test_an_empty_library_keeps_every_term() -> None:
     connection = _library([])