feat: interactive compare session

[danielcherubini]

Summary

• betterbench compare with no arguments now opens a temporary, loopback-only (127.0.0.1, kernel-picked port) GET-only browser session over $BETTERBENCH_HOME/runs: gallery of all runs (skipped/corrupt/ab-only dirs stay visible as muted rows), /run/<slug> report pages (the on-disk render_html output + an injected session bar), and a pair page with an A/B toolbar, an always-on "unpaired in time" banner, mismatch chips (corpus/sampling/host/GPU/notes), and a comparison band with paired 95% decode CIs (pass-index pairing, truncated) + median-only latency/prefill/concurrency deltas + combined decode (each run's own weights). No files written, per-request re-scan (a run landing mid-session appears on the next click), three distinct plain-text 404 forms, slug allowlist = per-request directory listing (no path probing/traversal).
• The two-arg betterbench compare a b terminal form keeps its existing behaviour verbatim (byte-pinned).
• betterbench/html_report.py split into render_sections + render_document so the session can reuse the exact report machinery — on-disk results.html output byte-identical (golden-pinned).
• Docs: README ## Compare your runs, METHODOLOGY ## Cross-run compare (CIs vs medians-only and why), CHANGELOG ## Unreleased.
• Rationale: docs/decisions/0001-compare-session-ephemeral-server.md; plan: docs/roadmap/compare-session.md.
• Review: 3-pass review loop — 13 actionable findings resolved (gallery degradation for malformed runs, encode-once/decode-once slug URL convention with %-named regression coverage, /run 404 fallback instead of connection reset, test protocol, doc nits). 158 tests green; ruff at the 17-item branch baseline (baseline 18 minus the E702 on the subparser semicolon line this PR's spec removes).

Test plan

• python -m pytest tests -q → 158 passed
• ~/.local/bin/ruff check betterbench/ tests/ → the 17-item branch baseline, no drift
• Manual: BETTERBENCH_HOME=<dir-with-a-run> betterbench compare → three lines printed, gallery in browser, pick two runs → pair page with banner/chips/band/iframes; Ctrl-C stops
• Manual: betterbench compare a.json b.json → table byte-identical to before
• Manual point-checks: a results.json-corrupt run dir renders as a muted gallery row and 404s with the error text on /run/<slug>; a run dir created while the session is open appears on the next gallery reload

[danielcherubini]