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.

frontend/src/components/MemoryPanel.tsx

@@ -103,6 +103,7 @@ export function MemoryPanel({ onChanged }: { onChanged: () => void }) {
       setNotice(
         `forgotten: ${report.atoms_deleted} atom${report.atoms_deleted === 1 ? '' : 's'}, ` +
           `${report.edges_deleted} links, ${report.cache_entries_cleared} cache entries` +
+          (report.semantic_cache_cleared ? `, ${report.semantic_cache_cleared} compiled contexts dropped` : '') +
           (report.vectors_dropped ? `, ${report.vectors_dropped} dense vectors dropped` : '') +
           (report.visual_index_dropped ? ', picture index dropped' : ''),
       );

frontend/src/types.ts

@@ -309,6 +309,7 @@ export interface ForgetReport {
   atoms_deleted: number;
   edges_deleted: number;
   cache_entries_cleared: number;
+  semantic_cache_cleared: number;
   vectors_dropped: number;
   visual_index_dropped: boolean;
   affected_cartridges: Array<Record<string, unknown>>;