fold: deterministic CreatedAt from persisted applied_at

[MSD21091969]

HG URN: urn:moos:session:sam.kernel-proper

Summary

• thread explicit timestamps through fold evaluation via EvaluateAt and EvaluateProgramAt
• use persisted rewrite time (AppliedAt with Timestamp fallback) during replay instead of wall clock
• align runtime apply/program/reactive/local_t paths so fold CreatedAt matches persisted AppliedAt
• strengthen replay determinism tests to assert full-node equality and timestamp fallback behavior

Why

Replay determinism previously depended on runtime wall clock for ADD/LINK CreatedAt, which made reconstructed state time-dependent. This changes fold state derivation so structural CreatedAt is deterministic from persisted log payload.

Finding provenance

• ffs0 design note: dev/design/manifold-bump-4_0/20260708-t249-fp-fruits.md (finding i: CreatedAt purity leak)
• ffs0 PR #144 baseline findings package: https://github.com/Collider-Data-Systems/ffs0/pull/144

Validation

• go test ./... (module root)

Notes

• HDC drift annotation re-home (Pre-existing: WF11 tagged/tagged-in HDC drift-claim LINK silently rejected by the declaration gate #52) is intentionally split into PR kernel: re-home HDC drift annotation from WF11 to WF15 #55.

[Copilot]

Pull request overview

This PR makes fold replay and runtime application deterministic by threading explicit timestamps through fold evaluation, so node/relation CreatedAt is derived from persisted log timestamps (AppliedAt, with Timestamp fallback) rather than the wall clock.

Changes:

• Add fold.EvaluateAt / fold.EvaluateProgramAt and update runtime paths to pass the exact persisted AppliedAt so CreatedAt matches the log.
• Update replay to use persisted timestamps (AppliedAt → Timestamp → zero time) to keep replay deterministic even for legacy/malformed log lines.
• Strengthen determinism tests to assert full node equality and validate the timestamp fallback behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/kernel/runtime.go	Threads a single captured AppliedAt into fold evaluation for Apply/ApplyProgram/reactive/local_t so persisted timestamps match fold CreatedAt.
internal/fold/replay.go	Uses persisted AppliedAt/Timestamp (else stable zero time) when folding during replay to avoid wall-clock dependence.
internal/fold/program.go	Introduces EvaluateProgramAt to accept caller-supplied per-envelope timestamps for deterministic CreatedAt in batch evaluation.
internal/fold/evaluate.go	Introduces EvaluateAt and updates ADD/LINK folding to use provided timestamps for CreatedAt.
internal/fold/fold_test.go	Updates CI-4 determinism test to compare full nodes and asserts CreatedAt preservation + timestamp fallback.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[MSD21091969]

Folded Copilot catch from #147 conventions brief:

• EvaluateProgramAt now validates appliedAt length strictly (0 or len(envelopes) only)
• partial slices now return an error instead of silently reintroducing wall-clock fallback
• added regression test: TestProgramAt_PartialAppliedAtRejected

Validation: go test ./... passed.
Commit: d7e18a4