fix: exception-safe aggregation backfill + generation-verified adoption + loud open-path guards

- Aggregation backfill walks build into a STAGING map and swap in atomically
  on completion. A mid-walk failure drops the staging map — the previous live
  state keeps serving, the aggregate stays flagged pending, and the storage
  error surfaces to the failing query. Previously the walk wiped live state
  before a scan that could throw, never cleared the pending flag on failure,
  and re-ran a full walk on every subsequent query: a silent wipe/walk/throw
  loop at the caller's retry rate.
- Failed walks are latched: retries within a 30s cooldown rethrow the recorded
  error instantly instead of re-walking, so a tight caller-side retry loop
  costs one loud error per query, never a full store walk per query.
- Persisted aggregation state is stamped with the store's committed generation
  at flush; reopen adoption requires stamp equality. Stale state (unclean
  shutdown) or over-counting state (a log truncation on a copied store pulled
  the watermark back) triggers exactly one loud rescan, never a silent adopt.
- The backfill/adoption path narrates: adoption decisions, walk start/finish
  with counts and duration, and failures all log by default.
- getNouns/getVerbs refuse a supplied-but-undecodable pagination cursor with a
  loud error instead of silently restarting the walk at offset 0 (which
  re-served page 1 forever to any while(hasMore) caller).
- The graph cold-load verb walk aborts loudly on a missing or non-advancing
  cursor with hasMore=true.
- A versioned index provider whose generation is AHEAD of the committed
  watermark (torn copy / crash-recovery truncation) is now named loudly at
  open, alongside the existing behind-direction message.

src/graph/graphAdjacencyIndex.ts

@@ -282,6 +282,16 @@ export class GraphAdjacencyIndex implements GraphIndexProvider {
       }
 
       hasMore = result.hasMore
+      if (hasMore && (!result.nextCursor || result.nextCursor === cursor)) {
+        // A stalled cursor with hasMore=true would re-read the same page
+        // forever — a silent full-CPU loop at cold open. Abort loudly; a
+        // graph read failing beats a process that spins without a log line.
+        throw new Error(
+          `GraphAdjacencyIndex: verb walk stalled after ${count} verbs — storage returned ` +
+            `hasMore=true with ${result.nextCursor ? 'a non-advancing' : 'no'} cursor. ` +
+            `Aborting the cold-load; run brain.repairIndex() if this persists.`
+        )
+      }
       cursor = result.nextCursor
     }