docs(headless): document v4 report rate semantics

[Astro-Han]

Summary

Documents the report-semantics change that #1758 (5c2bbc6db) introduced but did not disclose: schema v4's top-level effectiveness pass rates are arm-local (each arm's passes over its own valid cells), not the v3 shared paired denominator. Runs with unilateral infra gaps therefore read differently between v3 and v4 reports.

• field-level comments on HarnessAbArmEffectiveness.passRate, candidateMinusBaseline vs pairedCandidateMinusBaseline, and the nonBudgetConditional section (diagnostic disclosure, never the headline)
• README's report paragraph updated (it still described "schema v3")

No behavior change; comments and README only.

Verification

• packages/headless suite: 1456 pass / 0 fail (report tests confirm no code drift)
• format/lint clean

Review focus

Wording accuracy against buildHarnessAbReport: arm-local numerators/denominators at harness-ab-report.ts:135-146, paired delta at summary.passRateDelta.