T-11.3: Add spend data layer

[Mkohler4]

Summary

• Adds explicit-column Store read APIs for llm_runs, including list, source-filtered list, and provider/model aggregation.
• Aggregates token totals with NULL token columns treated as 0 while exposing hasUnknownTokens.
• Adds a bounded SpendModel that attaches T-11.2 CostEstimate rows, totals known-price USD only, and carries the price table stamp.
• Adds in-memory Store and SpendModel tests for grouping, NULL token handling, unknown-price totals, empty model state, and bounded rows.

Guardrails

• No UI changes.
• No schema migration and no Store/Schema.swift changes.
• No engine/ or mcp/ changes.
• Store read paths do not select cost_usd; app-side recordLLMRun leaves cost_usd untouched.

Verification

• swift build
• swift test (275 tests, 1 skipped, 0 failures)

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