fix: price gain estimates from resolved model

[malochapo]

Summary

Fixes #1243.

lean-ctx gain --deep now prices cost estimates from the best available model signal instead of falling straight back to blended pricing when env vars are absent.

This PR:

• carries the resolved model_key and pricing match kind through CostModel
• resolves gain pricing from env/config/proxy dominant model before fallback
• shows the resolved model and whether the price is exact/live/estimated/fallback in the cost breakdown
• makes usd_estimate reuse the same CostModel pricing path

Test plan

• cd rust && cargo test --lib pricing -- --nocapture
• cd rust && cargo fmt --check
• cd rust && cargo clippy --all-features -- -D warnings
• cd rust && cargo test -- --test-threads=1 (the suite shares process-global state; CI serializes it too)

Notes for reviewers

• Risk areas / edge cases: StatsStore remains aggregate, so this improves the dashboard estimate using the best current model signal; it does not reconstruct exact historical per-call model costs.
• Backwards compatibility: fallback blended pricing remains the final fallback when no model signal exists.
• Docs updated (links/files): no docs file changed; the cost breakdown label now surfaces the pricing source.

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