PLT-454: LoadTx lifecycle timestamp plumbing

[bdchatham]

Implements PLT-454 — the foundation PR of the coordinated-omission fix for the sei-load workload modeler.

What

• Add IntendedSendTs + reserved AttemptedSendTs / InclusionTs to LoadTx (types/scenario.go).
• Stamp IntendedSendTs in Dispatcher.Run/RunBatch right before hand-off, where the dispatcher is the sole owner of the pointer (race-free, written exactly once).
• Document the timestamp concurrency contract on the struct: single-writer-before-handoff, immutable after enqueue, InclusionTs owned solely by the future PLT-459 tracker.

Not in this PR (by design)

Nothing reads these fields for latency yet — the existing dequeue-relative path is untouched, so this is additive and behavior-preserving. PLT-458 moves IntendedSendTs to the true scheduled instant t₀ + i/λ; PLT-459 populates InclusionTs.

Tests

go build ./... clean; go test -race ./... all packages ok.

Decision brief: designs/sei-load-workload-modeler/PLT-454-loadtx-timestamps.md (sei-protocol/bdchatham-designs).

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[cursor]

PR Summary

Low Risk
Additive fields and one-time writes at existing ownership boundaries; no consumers or metric logic changes in this PR.

Overview
Adds lifecycle timestamp plumbing on LoadTx as the first step toward fixing coordinated omission in sei-load metrics.

LoadTx gains IntendedSendTime, AttemptedSendTime, and a reserved InclusionTime, plus documented single-writer / handoff concurrency rules (zero time = untracked). The dispatcher stamps IntendedSendTime immediately before enqueue in Run and RunBatch; workers stamp AttemptedSendTime at dequeue before the HTTP send. Nothing reads these fields yet—existing latency stats stay on the dequeue-relative path; IntendedSendTime is enqueue time today, not true scheduled t₀, until follow-up work.

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[bdchatham]

Expert review — converged (no remaining concerns)

Reviewed by independent specialists (not the authoring agent), two rounds:

• Idiom (Go): ✅ resolved — renamed …Ts→…Time to match the package's existing time.Time convention (startTime, lastWindowTime); removed ticket-number/rollout-state changelog prose; concurrency-contract doc anchored to the real txChan/sentTxs channels.
• Measurement semantics: ✅ resolved — IntendedSendTime now documents that it currently holds the back-pressured enqueue time and must not be used to derive latency until the open-loop scheduler sets it; ownership-transfer model corrected; zero=untracked documented. AttemptedSendTime is now stamped at the actual send attempt (was a dead field).

Deferred to PLT-458 (open-loop scheduler), recorded on that issue: reserve the per-tx sequence index i/epoch t₀ for the t₀ + i/λ schedule; account for dropped-tx zero-InclusionTime holes in any inclusion-rate denominator.

Behaviour-preserving (existing latency path untouched). go build + go test -race ./... green; the probabilistic TestAccountPoolMixedRate is a pre-existing unseeded-RNG flake (passes 12/12 under -count=1), addressed by PLT-456.