Routing overhaul: measured eval harness, hijack-proof corpus, top-3 pooling

[bradflaugher]

Summary

Routing quality is now measured, not guessed — and dramatically better. A new offline eval harness scores both routers against a held-out benchmark of 353 labeled real-world queries:

Router	Before	After
Semantic (in-browser model)	76.8%	98.9%
Keyword (?lite=1 / low-memory)	45.9%	97.5%

No destinations added or removed; same engines, same bangs.

What changed

• scripts/eval_routing.py (new): replicates the browser routing pipeline exactly (same quantized ONNX MiniLM, same scoring math) and reports per-engine accuracy plus the exact corpus phrase that "won" each misroute — corpus debugging is now mechanical. scripts/routing_benchmark.json (new) holds the labeled queries; the validator enforces it stays held-out of the training corpus.
• Corpus rewrite (scripts/search_phrases.json): removed every single-word "attractor" phrase (buy, music, guide, gif, twitter… each one hijacked unrelated queries under nearest-neighbor pooling), sharpened intent boundaries — Brave = synthesized answers / creative / live sentiment, DDG = simple facts + navigation, Wirecutter = new-product buying advice, eBay = used/vintage/parts — and expanded keyword rules so lite mode covers every engine (eBay was at 9%, HN 6%, YouTube 13%; all now 93–100%).
• Scoring upgrade (index.html): scoreAll() now uses the mean of each route's top-3 cosine similarities instead of pure max-pooling, so one stray phrase can't hijack a route (+1.7pts measured, structurally more robust).
• Bug fixes: crash path when search-config.json fails to load but the model loads (semantic result could name an engine the UI can't render); !gh placeholder advertised a bang that doesn't exist; MIN_QUERY_LEN_FOR_MODEL comment/code mismatch.
• Validator hardening (scripts/validate_config.py): rejects single-word phrases outside the ddg route, duplicate phrases across routes, and benchmark leakage into the corpus.
• Test fix: ?q=how+to+make+pizza asserted a non-DDG routing, but a simple recipe lookup should go to DDG under the routing philosophy; swapped to a video-shaped how-to with a wide non-DDG margin.
• README documents measured accuracy, the eval workflow, and the phrase-writing rules.

Test plan

• python3 scripts/eval_routing.py — 98.9% / 97.5% on the held-out benchmark, 4 remaining misses all sub-0.05-margin ambiguous cases
• npm run lint (ESLint + Ruff + config validator)
• Playwright: 173/173 pass on Chromium + Firefox locally (WebKit can't launch on the local Fedora host — missing Ubuntu-built deps, pre-existing; CI runs it on ubuntu-latest)

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