[Feature] Add typed success estimates and immutable evidence snapshots

[sujal111]

Add typed success estimates and an immutable in-memory evidence snapshot under Router Learning (#3480).
Observe mode records the estimates in Router Replay. The selected model does not change.
routing_sampling stays the default — this PR does not add success_constrained or apply mode.

Closes #3480
Related #3412
Related #3494

Purpose

First implementation PR after the proposal. pkg/extproc now freezes the in-memory experience at request start and reports a typed estimate per candidate (calibrated / insufficient_evidence / unsupported / stale / conflict). Seed-only QualitySeed and classifier/similarity scores are not treated as P(success).

wg/mom-routing

Next PRs

• [Feature] Add calibrated success-constrained model selection #3412: observe-only success_constrained (feasibility filter, lifecycle-cost ranking, fallback, decision-local success config). Still observe-only.
• Docs + config reference once that YAML is settled.
• Apply mode later, and only after [Feature] Build Router Learning experience materialization #2240, [Feature] Add adaptive thresholding and policy calibration #2341, and [Research] Compare Router Learning algorithms under a shared benchmark protocol #2346.

Test Plan

• go test ./pkg/extproc/ -run 'SuccessEstimate|TestRouterLearning|TestBuildReplayRoutingRecordCapturesLearning'
• Covers seed-only, missing calibration, stale evidence, sparse backoff, concurrency, and observe mode keeping the base model

Test Result

Targeted extproc run passed locally. Full make test-semantic-router is on CI — I could not rebuild the candle bindings here.

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[Xunzhuo]

One core part of #3480 is not wired into the observe path yet. attachSuccessEstimateObserveDiagnostics calls estimateCandidateSuccess with no stale horizon, so StaleAfter stays zero and stale evidence can never produce the stale result. The same path uses resolveExperience, which simply picks the first populated scope; mergeScopedExperience is never called, so conflicting scopes are silently accepted and the conflict result is unreachable. The direct helper tests do not exercise these production paths. Please wire the decision/global observe controls through the real lookup path and add observe-path coverage showing stale and conflicting evidence fail conservatively.