fix(routing): close ccmax cache cost feedback loop

[SakuraPuare]

Summary

Long-running sessions kept re-attributing cache cost back into itself, so upstreams that failed early or occasionally still showed up as cheap. This PR closes that loop with three focused changes:

1. Rolling cache creation counts and token volumes — cache reads and writes can overlap within one response; pricing now tracks both concurrently instead of collapsing to a single write-per-miss figure.
2. Persist per-attempt actuals — rejected route decisions and the real per-attempt cost are stored, and low-sample failures get a small penalty so unavailable or repeatedly failing upstreams remain explainable in the UI.
3. Focused test coverage — new tests for routing / store / audit paths (new file: internal/server/routing_audit_test.go).

Files touched (12)

• internal/forward/forward.go
• internal/routing/cachestate.go
• internal/routing/cost.go
• internal/routing/cost_test.go
• internal/routing/selector.go
• internal/routing/selector_test.go
• internal/routing/types.go
• internal/scheduler/intelligent.go
• internal/server/routing_audit.go
• internal/server/routing_audit_test.go (new)
• internal/store/routing_observations.go
• internal/store/routing_persistence_test.go

Validation

Rebased on top of current main (post-#19). On our CI runner:

go vet ./...     # clean
go test ./...    # all packages ok

Fork context: this PR carries the routing/cost fix on top of the InputInflation three-part accounting already in main; the cachedPortion/writeTokens decomposition preserves that accounting and adds the rolling create-count and observedWriteTokens override on top.