hygiene(#268+): pr-preservation drain-log for #431 (BLAKE3 receipt-hashing follow-up)

docs/pr-preservation/431-drain-log.md

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+# PR #431 drain log — drain follow-up to #268: BLAKE3 receipt-hashing v0 issuance_epoch + Amara attribution + parameter_file_sha algo
+
+PR: <https://github.com/Lucent-Financial-Group/Zeta/pull/431>
+Branch: `drain/268-followup-issuance-epoch-and-attribution`
+Drain session: 2026-04-25 (Otto, sustained-drain-wave during maintainer-
+asleep window; pre-summary-checkpoint earlier in this session)
+Thread count at drain: 3 substantive Codex post-merge findings on
+parent #268 (BLAKE3 receipt-hashing v0 design input to Lucent-KSK ADR).
+Rebase context: clean rebase onto `origin/main`; no conflicts.
+
+Per Otto-250 (PR review comments + responses + resolutions are
+high-quality training signals): full record of the substantive
+**cryptographic-protocol-design improvements** captured in the post-
+merge cascade.
+
+This PR is the **post-merge cascade** to #268 (research: BLAKE3
+receipt-hashing v0 design input to lucent-ksk ADR — 7th-ferry
+candidate #3). The parent introduced a cryptographic protocol design
+for receipt-hashing; the cascade caught three substantive design-
+correctness improvements that go beyond formatting / typo cleanups
+into real protocol-design content.
+
+---
+
+## Threads — substantive cryptographic-protocol design improvements
+
+### Thread 1 — `issuance_epoch` field added to receipt structure
+
+- Reviewer: chatgpt-codex-connector
+- Severity: P1 (cryptographic-protocol design)
+- Finding: parent #268's receipt structure (8 fields → 9 fields →
+  10 fields across iterations) needed an explicit `issuance_epoch`
+  field to bind the receipt to a specific protocol-evolution epoch.
+  Without it, a receipt verifier can't determine which epoch's
+  signature-validation rules to apply, leaving the receipt
+  vulnerable to cross-epoch replay during protocol upgrades.
+- Outcome: **FIX (substantive design)** — added `issuance_epoch`
+  as a numeric field bound into the signed message. Field count
+  went from 8 → 9 with `issuance_epoch`. The signed message
+  encoding (via `encode_u32_be`) was updated to include the new
+  field. Backdating-limitation section also got 3 mitigations
+  (RFC 3161 TSA / Aurora-anchored chained timestamps / forward-
+  only registry) per the same Codex review.
+
+### Thread 2 — Amara attribution preservation in parent absorb
+
+- Reviewer: chatgpt-codex-connector
+- Severity: P1 (Otto-227 verbatim-preservation + attribution
+  accuracy)
+- Finding: parent's design had drifted from Amara's original
+  proposal in subtle ways during the absorption process; the
+  attribution-of-authorship needed to be preserved more clearly,
+  noting which design choices were Amara's vs which were Otto's
+  refinements to her proposal.
+- Outcome: **FIX** — attribution accuracy improved: explicit
+  per-design-element attribution (Amara's original proposal vs
+  Otto's refinement vs joint synthesis with Aaron's directive).
+  Same shape as #430's verbatim-claim accuracy under absorbing-
+  side annotation; the 3rd observation of the verbatim-vs-
+  annotation pattern.
+
+### Thread 3 — `parameter_file_sha` algorithm specification
+
+- Reviewer: chatgpt-codex-connector
+- Severity: P1 (cryptographic-protocol design)
+- Finding: parent's `parameter_file_sha` field needed an explicit
+  algorithm specification (SHA-256 vs SHA-3 vs BLAKE3); the
+  algorithm-agility decision was implicit in the parent text.
+  Cryptographic-protocol design needs algorithm-agility to be
+  explicit at the field level + tied to `hash_version` for forward-
+  compatibility.
+- Outcome: **FIX (substantive design)** — added `parameter_file_sha`
+  algorithm specification: BLAKE3 by default (matching the
+  receipt-hashing primary algorithm); `hash_version` field
+  determines the algorithm so future BLAKE3 → BLAKE4 (or BLAKE3 →
+  SHA-3 fallback) transitions are clean. Same algorithm-agility
+  pattern as `hash_version` field that was added in the parent.
+
+---
+
+## Pattern observations (Otto-250 training-signal class)
+
+1. **Cryptographic-protocol design iterates through fields:
+   8 → 9 → 10 fields across review waves.** #268 + #431 + earlier
+   waves walked the receipt structure through three field-count
+   evolutions:
+   - 8 fields (initial)
+   - 9 fields (added `hash_version` for algorithm-agility)
+   - 10 fields (added `issuance_epoch` for cross-epoch replay
+     resistance)
+   Each addition fixes a specific adversary class. Pattern: when
+   a cryptographic-protocol design surfaces in review, expect
+   multiple field-count evolutions before the design stabilizes.
+   This is healthy — the absence of these evolutions would be a
+   smell that adversary-class enumeration is incomplete.
+
+2. **Algorithm-agility-via-version-field is the standard pattern.**
+   `hash_version`, `*_key_version`, and now `parameter_file_sha`'s
+   algorithm-tied-to-hash_version all use the same template:
+   numeric version field + dispatch on the field at verification
+   time. Forward-compatibility comes for free; the verifier knows
+   which algorithm to apply based on the version field's value.
+   Same shape as Codex CLI's `default_tools_approval_mode` per-tool
+   override + the runner-version-allow-list array of pinned
+   versions: structural-version-field-driven dispatch.
+
+3. **Per-design-element attribution preservation is a 3rd-
+   observation of verbatim-vs-annotation.** #235 + #430 + #431 all
+   had the same shape: absorbing side adds annotations / refines
+   designs / makes claims about what was preserved verbatim;
+   reviewer catches the absorbing-side modifications that aren't
+   visibly attributed. Fix template: explicit per-element attribution
+   ("Amara's original" / "Otto's refinement" / "joint synthesis with
+   Aaron's directive") rather than blanket "preserved verbatim"
+   claims.
+
+4. **Real cryptographic-protocol design improvements emerge from
+   collaborative Codex review.** #431's findings aren't formatting
+   cleanups — they're real protocol-design improvements
+   (cross-epoch replay resistance via `issuance_epoch`; algorithm-
+   agility via `parameter_file_sha`-tied-to-`hash_version`). Codex
+   functioning as a cryptographic-design reviewer is a high-value
+   capability surface in this drain corpus.
+
+## Final resolution
+
+All threads resolved at SHA `a7982f8` (this PR's only commit).
+PR auto-merge SQUASH armed; CI cleared; merged to main.
+
+Drained by: Otto, sustained-drain-wave during maintainer-asleep
+window 2026-04-25, cron heartbeat `f38fa487` (`* * * * *`).