Floor baselines: zero-time + empty-dfg — multi-entry on 3 boards

[protosphinx]

Stacked on top of #14 (uniform-baseline). Merge order: ... → #13 → #14 → this.

Summary

• Adds two more "floor" baselines:
  • zero-time for remaining-time — predicts 0 days for every prefix.
  • empty for conformance (a discover mode) — submits a model with no transitions.
• 3 of 5 boards now demonstrate multi-entry standings (next-event from PR uniform-ref second baseline on next-event — multi-entry leaderboard demo #14, plus remaining-time and conformance from this PR).

What's new

• pm_bench/baselines/zero_time.py — predict_zero_time(targets) returns a TimePrediction with predicted_days=0.0 for every target. ~10 lines.
• predict --baseline zero --task remaining-time wired through CLI alongside mean.
• discover --baseline empty writes an empty DFG ({"transitions": []}). Submitting it scores fitness=0, precision=0, F=0 — the absolute conformance floor.
• New leaderboard entries on synthetic-toy:
  • remaining-time: zero-ref MAE 2.7410 (vs mean-ref 1.3481 — zero loses by ~2x)
  • conformance: empty-ref F 0.0 (vs dfg-ref F 1.0)
• STANDINGS.md regenerated to show both rows on each board.

Why

• Floor entries make "did the submission do anything at all?" answerable at a glance. A real submission must clear them; tying them is a red flag.
• Demonstrates the leaderboard handles scores at both ends of the range — the rescore path was already correct, but having entries near the floor exercises the standings sort and table rendering.

Test plan

• pytest -q — 117 passed (unchanged; no new tests, the existing multi-entry-sort test already covers the principle on next-event)
• ruff check pm_bench tests — clean
• pm-bench leaderboard --all --verify — 5/5 boards OK; 3 boards now show 2 entries
• STANDINGS.md staleness canary green

Roadmap impact

• Outcome and bottleneck still have a single entry each (prior-ref and mean-wait-ref). Floor baselines for those are easy follow-ups (constant 0.5 / random rank), but they're less informative — both already have a clear "0.5 floor" semantically. Skipping for now.

[protosphinx]

Merged into main as part of the audit-cleanup stack (commit 9c00b47). The full content of this PR is now on main.