Problem

Ahrefs flagged 3 soft-orphan URLs on `/benchmarks/perp-fees`:

• `/benchmarks/perp-fees/eth`
• `/benchmarks/perp-fees/btc`
• `/benchmarks/perp-fees/sol`

All return 200, all listed in sitemap, but zero inlinks. Cause:

• `perp-fees.yml` defines `dimensions.chain` values as `ETH`, `BTC`, `SOL` (uppercase asset codes for Prom label injection)
• Parent page `PerChainPagesNav` rendered `href=/benchmarks/perp-fees/ETH` straight from `c.value`
• Sitemap emits lowercase via `canonicalChainSlug`
• `matchesChainSlug` is case-insensitive so both `/ETH` and `/eth` returned 200 → duplicate content + lowercase orphans

Fix

• `PerChainPagesNav` (parent → chain) + `SiblingChains` (chain → sibling) hrefs now use `canonicalChainSlug(c.value)` so internal links match the sitemap canonical
• `[chain]/page.tsx` 308s non-canonical casings + legacy aliases (e.g. `/ETH → /eth`, `/ton → /gram`) to the canonical lowercase slug, also consolidating external links

YAML values stay uppercase (Prom label injection unchanged).

Test plan after `vercel --prod`

• `curl -I /benchmarks/perp-fees/ETH` → 308 to `/eth`
• `curl -I /benchmarks/perp-fees/eth` → 200
• View source of `/benchmarks/perp-fees` → hrefs are lowercase
• `curl -I /benchmarks/l1-finality/ton` → still 308 to `/gram` (existing alias still works)