feat(integrity): per-bench expected_n + sample-health badge

[Flotapponnier]

Companion to #628 (same change, applied directly against main's
pre-refactor structure because main and dev have diverged in the
materialize layer).

Bench pages currently rank providers based on sample counts that
vary from 3 to 1500+ per provider depending on the bench cadence,
without surfacing the confidence level. The methodology page
advertises n >= 1000 which is misleading on low-cadence benches
like oracle-deviation (3 to 4 samples per provider) and perp-funding
(3 samples per venue by design).

This change introduces a per-bench expected_n declared in the YAML
spec. The page computes health = actual / expected_n per provider
and badges rows below 50 percent of expected as "low sample" or
hides rankings below 10 percent as "insufficient". The bench-wide
aggregate also propagates to /api/citable, /api/stat and the
headline sentence so an "insufficient" field stops asserting a
winner in machine-readable feeds.

YAML field is optional. Benches without expected_n behave as before
(no badge, no hide).

YAMLs touched: oracle-deviation (4), perp-funding (3),
aggregator-head-lag (50000), bridge-quote-latency (3000),
bridge-fee (1000).

Test plan:

• pnpm validate (26 specs on main, passes)
• pnpm typecheck (clean)
• pnpm build (succeeds)
• bun test src/lib (56/56 pass, includes sample-health.test.ts)

[Flotapponnier]

Conflicts with #627 (insufficient propagate, already merged to main). Main already serves the integrity story via isInsufficient on /api/stat /api/citable /api/llm-context /api/mcp + bench card pill + home table pill. The low-sample BADGE UI added by this PR is a refinement on top, not critical. Will rebase and ship in a follow-up if needed, or via a future wholesale dev to main merge. Dev has the badge live via #628.