fix: validate benchmark observations and document composition candidate

[Pigbibi]

Summary

• Resolve the benchmark guard's observation date and calendar-day age from its own last numeric non-null price at or before the requested cutoff. A newer unrelated symbol can no longer refresh stale benchmark data.
• Park the guard with the existing benchmark_price_invalid reason when any price in its consumed trailing lookback is nonfinite or nonpositive. Invalid prices are not dropped or backfilled; values outside the cutoff/lookback are not consumed.
• Add a design-only same-dimension constraint-composition candidate. The document is explicitly DESIGN_ONLY_NOT_RUNTIME, depends on a separate strong no-add/release contract, and does not implement or enable a new arbiter.

Scope and compatibility

No thresholds, risk scalars, arbitration priorities, dependency pins, schema changes, runtime wiring, order permissions, or live activation. Existing missing-history reason codes and validation precedence are preserved. as_of=None retains the latest matrix date as the requested cutoff, while the guard's emitted observation date is benchmark-specific. Duplicate-date normalization remains unchanged and is a separate policy ambiguity.

The current selected UES consumer already prevalidates QQQ; this helper defect is not evidence of a live consumer failure.

Validation

• Freshness RED: 16 failing regression cases before the fix
• Invalid-price RED: 22 failing regression cases before the bounded validation fix
• Focused guard tests: 87 passed, including long/wide prices, all-missing and future-only inputs, missing cutoff observations, age boundaries, future-data exclusion, nonfinite/nonpositive positions, and unified fail-closed arbitration
• Full offline suite: 333 passed; one pre-existing local SDK installation-provenance failure (direct_url.json lacks vcs_info) also reproduced on untouched main
• Exact payload equality for 45 unaffected compatibility cases
• Ruff 0.15.9 passed locally; CI must verify the declared 0.16.9 pin
• Patch whitespace/applicability check passed; direct setuptools wheel and sdist backend builds passed offline

Local checks disabled socket connections and made no provider, AI, real-data, broker, or order calls. The exact declared AI dependency provenance and python -m build frontend still require the repository's clean CI environment. This remains a draft for review.