session-memory: denoise error fingerprints (WP-0006 follow-up)

Tighten _is_failed: exclude successful hub JSON responses (top-level no-error
payloads) and file-read snapshots (numbered cat -n source lines) that were
polluting error_snippets. JSON verdict classifies error vs success payloads
directly. Cuts distinct fingerprints 444 -> 269 (~40%) over the real corpus with
the top errors unchanged. Assessment caveat updated. 5 new tests; suite 102/102.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ASSESSMENT-infra-friction.md

@@ -120,11 +120,14 @@ Reading:
   in 3 sessions each — corroborates the plumbing-overhead story and the live MCP
   flakiness seen during this work (REST fallback used).
 
-**Caveat — fingerprint noise:** the fail-hint heuristic also catches non-failures
-(successful hub JSON responses, source lines containing `raise …Error`, linter
-"N errors" summaries). The *top* fingerprints above are real; a future refinement
-should tighten `_is_failed` (e.g. skip valid-JSON success payloads and code-read
-snapshots) before trusting the long tail.
+**Fingerprint noise — mostly handled.** `_is_failed` now excludes successful hub
+JSON responses (top-level no-error payloads) and file-read snapshots (numbered
+`cat -n` source lines), which cut distinct fingerprints **444 → 269 (~40 %)**
+without touching the top entries. Residual low-value items remain in the long tail
+(bare structural lines like `{`, linter "N errors" summaries); the *top*
+fingerprints are real. Note several entries (`MCP error -32602`,
+`update_task_status 'title'`) reflect the State Hub MCP instability hit live during
+this work — genuine, if self-referential, friction.
 
 ## What this assessment still can't see