fix(consider): preserve mixed-currency valuation frame

[atomchung]

What

• bind the already-normalized native closes and USD-per-unit FX to the exact canonical holdings through the existing ValuationFrame builder
• pass that complete frame into the existing PortfolioBasis sizing reader for mixed-currency ledger-backed consider
• preserve the established single-currency legacy receipt unchanged
• keep recoverable currency/FX/sizing refusals outside [design·P1] When consider cannot compute, no contract owns the answer — so the agent narrates the gap instead of framing the decision #674's bounded whole-book usable_facts lane

Why

consider resolved usable native closes and FX, then reduced the feed to {as_of, prices} before canonical sizing. That discarded currencies and fx_to_aggregate, so the existing canonical reader correctly refused a mixed-currency book even though the call already had every fact it needed.

User impact: a valid mixed-currency pre-trade question now reaches engine-owned before/after weights and rule collisions instead of entering unnecessary recovery.

Safety boundaries

• reuses the existing price_feed, ValuationFrame, and PortfolioBasis owners
• no second sizing calculation, agent arithmetic, identity-FX fallback, provider abstraction, migration, or durable-state change
• frame prices are restricted to exact current holdings; premise and closed-symbol extras cannot widen it
• supplied holding-currency conflicts refuse by ticker and currencies
• missing TWD FX refuses by name and never evaluates on cost basis
• public fixtures and evidence are synthetic

Verification

• PYTHONPYCACHEPREFIX=/tmp/pr712-pycache-full python3.12 tests/run_all.py — all 52 offline suites passed
• tests/test_consider.py — 131/131
• tests/test_portfolio_basis.py — 26/26
• tests/test_consequence.py — 57/57
• focused E2E covers both auto-resolved and --prices USD/TWD ledger paths, aggregate before/after weights, and a would_breach rule collision
• isolated mutation restoring the old {as_of, prices} downgrade makes both new composed E2E tests fail with the original canonical sizing refusal
• independent read-only review: no block findings after two rounds; the first round's currency-conflict and named-missing-FX findings are covered by CLI regressions

Closes #712

[atomchung]

Independent review completed in two rounds against the exact PR commit. Round 1 found two correctness gaps: a supplied holding-currency conflict could be relabelled into the ledger currency, and a pure missing-FX refusal did not name the missing currency. Both now have real CLI regressions, write no evaluation, and remain outside #674's usable_facts lane. Round 2 found no block issues. Local Python 3.12 offline suite: 52/52; intentional restoration of the old {as_of, prices} downgrade kills both new mixed-currency composed E2E tests. GitHub's Python 3.11/3.12 matrix remains the final merge gate.