fix(seo): 404 hex compare pages + add curated pairs to sitemap

[Flotapponnier]

Why

Ahrefs flagged 1690 `/compare/-vs-` pages as "indexable but not in sitemap" — symptom of two distinct problems.

Pattern analysis

• ~70% `0xHEX-vs-` (ex: `0xb290f2f3-vs-liquid-perps`): HL builder addresses auto-generated as ad-hoc compare slugs. Zero search demand, pure crawl-budget waste. → fix drop journal framing #1
• ~20% chain-vs-chain (ex: `aptos-vs-polygon`): kept as-is for now (some have real search demand, will revisit if traffic data justifies dropping).
• ~10% legit provider-vs-provider (ex: `meowrpc-vs-publicnode`, `covalent-vs-mobula`): curated in `compare-pairs.ts` but missing from sitemap. → fix publish harnesses #2

Changes

1. `resolveAdHocPair()` in `src/app/compare/[slug]/page.tsx` now rejects slugs matching `/^0x[0-9a-f]{4,}$/i` on either side. `/compare/0xb290f2f3-vs-*` → 404. Verified no curated pair uses hex slugs.
2. `buildFullSitemap()` emits one entry per `COMPARE_PAIRS` entry (21 hand-picked pairs), each defensively re-validated via `getProvider` to skip dead pairs.

Chain-vs-chain comparisons kept — separate review needed if we want to dedupe them against `/chains` hubs.

Flow

Push dev → cherry-pick main.