Add metric design walkthrough page

[MaxGhenis]

Summary

Adds a standalone HTML thinking-doc at `/metric-options.html` that walks through scoring options for the multi-output benchmark, step by step on a stylized example (3 households × 5 outputs).

The walkthrough builds up:

• A continuous row score with no $1,000 floor: `max(0, 1 - |pred-ref| / |ref|)` and `ref=0 → 1 iff pred=0`. The `max(0, …)` already caps the score at 0 when the error exceeds the reference, so the floor wasn't doing extra work. Booleans (`ref ∈ {0, 1}`) drop out at 0 or 1 naturally — no special branch needed.
• Equal weights, per-household impact, 30% floor (current), global variable weights (recommended), two-score breakdown (amount + participation), five ways to combine A and P (arithmetic, geometric, harmonic / F1, min, weighted), budget-weighted, net-income reconstruction, and a brief "other variants we considered" section.
• Booleans like medicaid_eligible stay in the same single-bucket scheme: the LLM is graded only on the eligibility flag (binary accuracy), and PE's paired `medicaid_value` supplies `|ref|` for the weighting schemes without the LLM ever having to predict the per-capita value.

Numbers are computed inline from one JS source of truth, so all 19 tables stay consistent.

Recommendation surfaced in the closing card: ship global variable weights as the headline, amount + participation as the diagnostic companion, harmonic mean (F1) of A and P only if a single combined column is non-negotiable.

This is a thinking doc, not a published metric. Follow-up PRs will implement the recommended metric in `analysis.py` and update the paper / app prose to match.

Test plan

• Page loads at `/metric-options.html` with no console errors.
• All 11 leaderboard rows render with consistent numbers across worked-example tables (e.g. amount accuracy: Tax 73.3%, Benefit 64.8%; participation: Tax 66.7%, Benefit 93.3%; global variable weights: Tax 65.3%, Benefit 82.1%).
• Scroll behavior is 1:1 (no header jank from the prior layout).
• Mobile layout collapses cleanly.

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Project	Deployment	Actions	Updated (UTC)
policybench-site	Ready	Preview, Comment	May 17, 2026 2:15pm