docs(benchmarks): reproducible 3-way comparison vs trafilatura + firecrawl (#25)

Replaces the previous benchmarks/README.md, which claimed specific numbers
(94.2% accuracy, 0.8ms extraction, 97% Cloudflare bypass, etc.) with no
reproducing code committed to the repo. The `webclaw-bench` crate and
`benchmarks/fixtures`, `benchmarks/ground-truth` directories it referenced
never existed. This is what #18 was calling out.

New benchmarks/ is fully reproducible. Every number ships with the script
that produced it. `./benchmarks/run.sh` regenerates everything.

Results (18 sites, 90 hand-curated facts, median of 3 runs, webclaw 0.3.18,
cl100k_base tokenizer):

  tool          reduction_mean   fidelity        latency_mean
  webclaw              92.5%    76/90 (84.4%)        0.41s
  firecrawl            92.4%    70/90 (77.8%)        0.99s
  trafilatura          97.8%    45/90 (50.0%)        0.21s

webclaw matches or beats both competitors on fidelity on all 18 sites
while running 2.4x faster than Firecrawl's hosted API.

Includes:
- README.md              — headline table + per-site breakdown
- methodology.md         — tokenizer, fact selection, run rationale
- sites.txt              — 18 canonical URLs
- facts.json             — 90 curated facts (PRs welcome to add sites)
- scripts/bench.py       — the runner
- results/2026-04-17.json — today's raw data, median of 3 runs
- run.sh                 — one-command reproduction

Closes #18

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

benchmarks/run.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# Reproduce the webclaw benchmark.
+# Requires: python3, tiktoken, trafilatura. Optional: firecrawl-py + FIRECRAWL_API_KEY.
+
+set -euo pipefail
+cd "$(dirname "$0")"
+
+# Build webclaw if not present
+if [ ! -x "../target/release/webclaw" ]; then
+    echo "→ building webclaw..."
+    (cd .. && cargo build --release)
+fi
+
+# Install python deps if missing
+missing=""
+python3 -c "import tiktoken"     2>/dev/null || missing+=" tiktoken"
+python3 -c "import trafilatura"  2>/dev/null || missing+=" trafilatura"
+if [ -n "${FIRECRAWL_API_KEY:-}" ]; then
+    python3 -c "import firecrawl" 2>/dev/null || missing+=" firecrawl-py"
+fi
+if [ -n "$missing" ]; then
+    echo "→ installing python deps:$missing"
+    python3 -m pip install --quiet $missing
+fi
+
+# Run
+python3 scripts/bench.py