[Rust port] relayburn-analyze: foundation (pricing + cost + fidelity)

[willwashburn]

Parent: #244
Depends on: #242 (reader types)

Context

Bootstrap the relayburn-analyze crate. This lands the scaffolding plus the three foundational modules that nearly every other analyze submodule consumes: pricing tables, per-record cost derivation, and the fidelity summary used to gate aggregations on input quality.

Scope

Crate scaffold

• Set up crates/relayburn-analyze/src/lib.rs with module declarations and the public re-export surface mirroring packages/analyze/src/index.ts (modules will be filled in by the follow-up sub-issues — for this PR, only pricing, cost, and fidelity are wired up).
• Add relayburn-reader as a path dep (already in workspace).
• Decide on the canonical numeric type for USD cost. TS uses number (f64); use f64 in Rust to keep arithmetic equivalent, but document the 1e-9 USD precision contract that the overhead sub-issue will need to honor.

pricing.rs (port of packages/analyze/src/pricing.ts, ~100 LoC)

• Port ModelCost, PricingTable, ReasoningMode types with serde::Deserialize.
• Port flatten(), loadBuiltinPricing(), loadPricing(override_path). The vendored models.dev.json snapshot from packages/analyze/data/ ships embedded via include_str! so the binary stays self-contained.
• loadPricing should accept an optional override path, falling back to the bundled snapshot.

cost.rs (port of packages/analyze/src/cost.ts, ~136 LoC)

• Port CostBreakdown, CostForUsageOptions types.
• Port costForUsage(), costForTurn(turn, pricing), lookupModelRate(model, pricing), sumCosts(costs).
• The costForTurn math (per-token rate × token count, with cache-read / cache-create / reasoning splits) is the precision-sensitive core — keep f64 ordering identical to TS so accumulation drift stays bounded.

fidelity.rs (port of packages/analyze/src/fidelity.ts, ~107 LoC)

• Port FidelitySummary struct, emptyFidelitySummary(), summarizeFidelity(), hasMinimumFidelity().
• This is small but it's the predicate the higher-level aggregators (compare, hotspots) use to refuse stats on undersized fixtures, so it needs to land before them.

Conformance gate

Port these test fixtures with equivalent Rust tests:

• packages/analyze/src/cost.test.ts (289 lines)
• packages/analyze/src/fidelity.test.ts (146 lines)

Pricing has no dedicated *.test.ts (it's exercised transitively by cost). The bundled models.dev.json snapshot must produce identical PricingTable lookups vs the TS path.

Acceptance

• cargo build -p relayburn-analyze green; crate exports pricing, cost, fidelity modules.
• cargo test -p relayburn-analyze green for the cost + fidelity test ports.
• costForTurn output matches TS to within 1e-12 USD on the cost-test fixture corpus (the headroom for the 1e-9 overhead gate later).
• Bundled models.dev.json snapshot is embedded; no runtime file dep.
• Public surface: loadBuiltinPricing, loadPricing, flatten, costForTurn, costForUsage, sumCosts, lookupModelRate, emptyFidelitySummary, summarizeFidelity, hasMinimumFidelity plus the listed types.

Files (TS source-of-truth)

• packages/analyze/src/pricing.ts
• packages/analyze/src/cost.ts + cost.test.ts
• packages/analyze/src/fidelity.ts + fidelity.test.ts
• packages/analyze/data/models.dev.json (bundled)