fix(template) + header redesign (CMC-style + sub-nav)

[Flotapponnier]

Summary

Follow-up to PR #671 (already merged). Two unrelated fixes on the same branch:

1. Template leak fix

• `{{best_name}}` / `{{best_p50}}` were leaking onto bench pages.
• `renderBenchmarkText` now also resolves `seoTitle`, `seoDescription`, `subtitle`, `methodology`, `disclaimer` (only `abstract` / `findings` / `seoIntro` / `faq` / `perChainExplainer` were covered before).
• Chain stash lookup is case-insensitive: perp-fees declares `dimensions.chain` as `ETH` / `BTC` / `SOL` (uppercase) so `{{best_p50:chain:BTC}}` previously fell through.
• Bumped cache keys `bench-unfiltered v12→v13` + `all-benchmarks v16→v17` so prod regenerates.

2. Header redesign (CMC-style)

• Removed the `|` separator between Contribute and GitHub.
• Active-section underline (accent color) under the current tab.
• 15px nav, taller bar (h-16 desktop), backdrop-blur on the sticky.
• GitHub icon-only, gap-spaced utilities, no pipe.
• New `SiteSubNav`: category tabs on `/benchmarks*` (All / Aggregators / Bridges / Blockchains / Trading / RPCs / Wallets / NFT APIs), hidden on other sections. Horizontal scroll on mobile with edge-fade mask.

Test plan

• Visit `/benchmarks/aggregator-head-lag` → no raw `{{best_name}}` in DOM, meta description, or view-source.
• Visit `/benchmarks/perp-fees/btc` → per-chain explainer renders the resolved value.
• Header: navigate Benchmarks → Products → About → confirm underline follows the active tab.
• Sub-nav: on `/benchmarks`, category tabs visible; on `/about`, no sub-nav strip.
• Mobile: hamburger menu still works; sub-nav scrolls horizontally with fade hint.