Improve Search Assist comparison recovery and browser resilience

[variablenix]

Summary

• Recognize cost and comparison questions, retaining both subjects in a bounded normalized Search Assist retry.
• Parse preload tags regardless of attribute order and decode Search Assist payloads without relying on adjacent JavaScript formatting.
• Wait for cited answer content rather than an empty browser heading, support expanded and short answer cards, and reserve fallback time.
• Serialize expensive browser lookups across networks and document timeout tuning and provider limitations.
• Keep existing entity fallbacks, but do not substitute an unrelated entity description for a comparison.

Verification

• go test -race ./...
• go vet ./...
• staticcheck ./...
• govulncheck ./...: no vulnerabilities found
• Offline Chrome and Chromium fixtures for delayed, expanded answers; local short-answer fixture
• Mocked recovery for the exact dirt-bike/go-kart query
• Live server smoke tests: exact comparison returned a cited Search Assist answer; existing definition and question cases passed

DuckDuckGo can still present an anti-automation challenge: this was observed on the browser-only route during verification. This change does not bypass challenges or guarantee identical results to an interactive browser.