fix(seo): unblock /products/us/<slug> detail pages (BUY-63952 P0)

[BuyWhere]

"## Problem (BUY-63952 P0)\n\nEvery US product detail URL at /products/us/<slug> returned HTTP 410 Gone (verified: https://buywhere.ai/products/us/asus-rog-zephyrus-g16 → 410, lenovo-legion-pro-5i → 410). This broke the search → product → buy funnel: inbound Google-indexed US product URLs and internal related-product links were all dead.\n\n## Root cause\n\nsrc/middleware.ts hard-410'd every single-segment /products/us/<slug> URL:\n\nts\n// Dead US product slug pages — same thin-content issue (BUY-40757)\nif (normalizedForDead.startsWith(\"/products/us/\")) {\n const afterUsPrefix = normalizedForDead.slice(\"/products/us/\".length);\n if (afterUsPrefix.split(\"/\").filter(Boolean).length <= 1) {\n return new NextResponse(null, { status: 410, ... });\n }\n}\n\n\nThis was an old SEO decision (BUY-40757) to de-index thin-content pages. But the single-segment route (src/app/products/us/[slug]/page.tsx) now SSR-renders real content — it resolves the product from the -<id> slug suffix via resolveUSProductRoute and renders USProductDetail + fetchUSProductSSR (live price comparison), falling back to notFound() (404) for genuinely unknown slugs. So the 410 was suppressing a functional page.\n\nNote: the QA report's claim that search-result cards link to /products/us/<slug> is inaccurate — SearchResultsClient.tsx links directly to affiliate/merchant URLs. The real exposure is (a) Google-indexed legacy URLs and (b) internal links from USProductDetail (related products) and ProductCard (share actions).\n\n## Fix\n\nStop 410'ing US single-segment slugs; let the page render. This mirrors the existing /about decision (BUY-58440): let the page render real content + metadata so Google can index it, instead of suppressing with a 410.\n\n- SG single-segment slugs stay 410'd — their page is still client-only thin content (the original rationale still holds; out of scope here).\n- The 2-segment canonical /products/us/<merchant>/<id> is untouched and remains the sitemap canonical.\n\n## Verification\n\n- tsc --noEmit: 0 new errors (197 pre-existing on clean main, unchanged). src/middleware.ts clean.\n- Logic harness confirming before/after path-classification (all assertions PASS):\n | Path | before | after |\n |---|---|---|\n | /products/us/asus-rog-zephyrus-g16 | 410 | pass→page |\n | /products/us/asus-rog-zephyrus-g16/ (trailing slash) | 410 | pass→page |\n | /products/us/<name>-<id> (real id-suffix form) | 410 | pass→page |\n | /products/us/amazon/123 (2-seg canonical) | pass→page | pass→page (unchanged) |\n | /products/sg/some-dead-slug (SG) | 410 | 410 (unchanged) |\n- getUSProducts() degrades gracefully (returns [] on API failure → page returns 404, not 500), so no risk of trading a 410 for a 500.\n- Could not run the full next dev middleware in this sandbox (EvalError: Code generation from strings disallowed — edge-runtime eval blocked by the Paperclip sandbox, affects all routes including /). Logic-level verification above covers the change; post-merge prod curl will confirm the 410 is gone.\n\n## Post-merge check\n\nbash\ncurl -s -o /dev/null -w \"%{http_code}\\n\" https://buywhere.ai/products/us/asus-rog-zephyrus-g16\n# expect: NOT 410 (200 real content for known product, 404 for unknown)\n\n\n🤖 Generated with Claude Code\n"