fix(brief): category-gated context + RELEVANCE RULE to stop formulaic grounding (#3281)

* fix(brief): category-gated context + RELEVANCE RULE to stop formulaic grounding

Shadow-diff of 15 v2 pairs (2026-04-22) showed the analyst pattern-
matching the loudest context numbers — VIX 19.50, top forecast
probability, MidEast FX stress 77 — into every story regardless of
editorial fit. A Rwanda humanitarian story about refugees cited VIX;
an aviation story cited a forecast probability.

Root cause: every story got the same 6-bundle context block, so the
LLM had markets / forecasts / macro in-hand and the "cite a specific
fact" instruction did the rest.

Two-layer fix:

  1. STRUCTURAL — sectionsForCategory() maps the story's category to
     an editorially-relevant subset of bundles. Humanitarian stories
     don't see marketData / forecasts / macroSignals; diplomacy gets
     riskScores only; market/energy gets markets+forecasts but drops
     riskScores. The model physically cannot cite what it wasn't
     given. Unknown categories fall back to all six (backcompat).

  2. PROMPT — WHY_MATTERS_ANALYST_SYSTEM_V2 adds a RELEVANCE RULE
     that explicitly permits grounding in headline/description
     actors when no context fact is a clean fit, and bans dragging
     off-topic market metrics into humanitarian/aviation/diplomacy
     stories. The prompt footer (inline, per-call) restates the
     same guardrail — models follow inline instructions more
     reliably than system-prompt constraints on longer outputs.

Cache keys bumped to invalidate the formulaic v5 output: endpoint
v5 to v6, shadow v3 to v4. Adds 11 unit tests pinning the 5
policies + default fallback + humanitarian structural guarantee +
market policy does-see-markets + guardrail footer presence.

Observability: endpoint now logs policyLabel per call so operators
can confirm in Vercel logs that humanitarian/aviation stories are
NOT seeing marketData without dumping the full prompt.

* test(brief): address greptile P2 — sync MAX_BODY_BYTES + add parseWhyMattersV2 coverage

Greptile PR #3281 review raised two P2 test-quality issues:

1. Test-side MAX_BODY_BYTES mirror was still 4096 — the endpoint
   was bumped to 8192 in PR #3269 (v2 output + description). With
   the stale constant, a payload in the 4097–8192 range was
   accepted by the real endpoint but looked oversize in the test
   mirror, letting the body-cap invariant silently drift. Fixed
   by syncing to 8192 + bumping the bloated fixture to 10_000
   bytes so a future endpoint-cap bump doesn't silently
   re-invalidate the assertion.

2. parseWhyMattersV2 (the only output-validation gate on the
   analyst path) had no dedicated unit tests. Adds 11 targeted
   cases covering: valid 2 and 3 sentence output, 100/500 char
   bounds (incl. boundary assertions), all 6 banned preamble
   phrases, section-label leaks (SITUATION/ANALYSIS/Watch),
   markdown leakage (#, -, *, 1.), stub echo rejection, smart/
   plain quote stripping, non-string defensive branch, and
   whitespace-only strings.

Suite size: 50 to 61 tests, all green.

* fix(brief): add aviation policy to sectionsForCategory (PR #3281 review P1)

Reviewer caught that aviation was named in WHY_MATTERS_ANALYST_SYSTEM_V2's
RELEVANCE RULE as a category banned from off-topic market metrics, but
had no matching regex entry in CATEGORY_SECTION_POLICY. So 'Aviation
Incident' / 'Airspace Closure' / 'Plane Crash' / 'Drone Incursion' all
fell through to DEFAULT_SECTIONS and still got all 6 bundles including
marketData, forecasts, and macroSignals — exactly the VIX / forecast
probability pattern the PR claimed to structurally prevent.

Reproduced on HEAD before fix:
  Aviation Incident -> default
  Airspace Closure  -> default
  Plane Crash       -> default
  ...etc.

Fix:
  1. Adds aviation policy (same 3 bundles as humanitarian/diplomacy/
     tech: worldBrief, countryBrief, riskScores).
  2. Adds dedicated aviation-gating test with 6 category variants.
  3. Adds meta-invariant test: every category named in the system
     prompt's RELEVANCE RULE MUST have a structural policy entry,
     asserting policyLabel !== 'default'. If someone adds a new
     category name to the prompt in the future, this test fires
     until they wire up a regex — prevents soft-guard drift.
  4. Removes 'Aviation Incident' from the default-fall-through test
     list (it now correctly matches aviation).

No cache bump needed — v6 was published to the feature branch only a
few minutes ago, no production entries have been written yet.

shared/brief-llm-core.js

@@ -144,17 +144,28 @@ export const WHY_MATTERS_ANALYST_SYSTEM_V2 =
   'on why the story matters.\n\n' +
   'STRUCTURE:\n' +
   '1. SITUATION — what is happening right now, grounded in a SPECIFIC named actor, ' +
-  'metric, date, or place drawn from the context.\n' +
+  'metric, date, or place relevant to this story.\n' +
   '2. ANALYSIS — the structural consequence (why this forces a repricing, shifts ' +
   'the balance, triggers a cascade).\n' +
   '3. (Optional) WATCH — the threshold or indicator to track, if clear from the context.\n\n' +
   'HARD CONSTRAINTS:\n' +
   '- Total length 40–70 words across 2–3 sentences.\n' +
   '- MUST reference at least ONE specific: named person / country / organization / ' +
-  'number / percentage / date / city — drawn from the context, NOT invented.\n' +
+  'number / percentage / date / city.\n' +
   '- No preamble ("This matters because…", "The importance of…").\n' +
   '- No markdown, no bullet points, no section labels in the output — plain prose.\n' +
-  '- Editorial, impersonal, serious. No calls to action, no questions, no quotes.';
+  '- Editorial, impersonal, serious. No calls to action, no questions, no quotes.\n\n' +
+  'RELEVANCE RULE (critical, read carefully):\n' +
+  '- The context block may contain facts from world-brief, country-brief, risk scores, ' +
+  'forecasts, macro signals, and market data. These are BACKGROUND — only cite what is ' +
+  "directly relevant to this story's category and country.\n" +
+  '- If NO context fact clearly fits, ground instead in a named actor, place, date, ' +
+  'or figure drawn from the headline or description. That is a VALID grounding — do ' +
+  'NOT invent a market reading, VIX value, or forecast probability to satisfy the rule.\n' +
+  '- NEVER drag an off-topic market metric, FX reading, or probability into a ' +
+  'humanitarian, aviation, diplomacy, or cyber story. A story about a refugee flow ' +
+  'does not need a VIX number; a story about a drone incursion does not need an FX ' +
+  "stress reading. If it isn't editorially connected to the story, leave it out.";
 
 /**
  * Parse + validate the analyst-path v2 LLM response. Accepts