feat: enforce MemoSize invariants at compile time and add DashMemo coverage

[QuantumExplorer]

Summary

Closes findings #2 and #3 from the MemoSize PR review (see #1 for context):

• MemoSize size invariants are now enforced at compile time. The trait documented — but nothing enforced — that NotePlaintextBytes = 52 + memo and NoteCiphertextBytes = 52 + memo + 16. A mis-sized third-party impl compiled cleanly and then panicked at runtime: at the expect in note_plaintext_bytes during encryption, and via unguarded enc.len() - 16 slicing inside the consensus-critical txid digest (hash_bundle_txid_data). The trait now carries a MEMO_SIZE associated const plus a SIZE_CHECK const assertion referenced by the encryption, decryption, and txid-hashing paths — a mis-sized implementation fails at monomorphization with an explicit message instead of panicking in production (verified manually: a copy-paste-broken impl fails to build with E0080: evaluation of SIZE_CHECK failed).

• First DashMemo test coverage in the crate. Previously no test instantiated M = DashMemo; the 88/104-byte code paths only ever ran in downstream consumers.

Changes

• src/memo.rs: const MEMO_SIZE: usize and const SIZE_CHECK: () on the MemoSize trait; consts on both impls; compile-time regression test
• src/note_encryption.rs: SIZE_CHECK forced in note_plaintext_bytes (encrypt) and split_plaintext_at_memo (decrypt)
• src/bundle/commitments.rs: SIZE_CHECK forced in hash_bundle_txid_data, guaranteeing the ciphertext slicing cannot panic; golden txid digest vectors for both memo sizes from fully deterministic bundles
• src/bundle.rs: Bundle<EffectsOnly, V>::from_parts generalized over M: MemoSize — it was pinned to the ZcashMemo default (same lock-in pattern as the PCZT impls fixed in fix: make PCZT trial decryption generic over MemoSize #6), making effects-only bundles unconstructible for Dash
• tests/dash_memo.rs (new): public-API round-trip for a 36-byte-memo bundle — full trial decryption, compact decryption, OVK output recovery, txid commitment — plus a deterministic golden vector pinning the 104-byte Dash note ciphertext wire format

The golden vectors double as the compatibility contract for any future migration (e.g. onto upstream's OrchardZSA flavor architecture): a byte-exact reimplementation must reproduce these exact digests and ciphertexts.

API note: adding a required associated const to the public MemoSize trait is a breaking change for external implementors of the trait (not for users of ZcashMemo/DashMemo) — intentional, and cheap to absorb pre-release.

Test plan

• cargo test — 86 tests pass (full suite including proving tests)
• Manual negative test: a mis-sized MemoSize impl fails to compile with E0080 on SIZE_CHECK
• cargo clippy --all-features --all-targets clean
• cargo fmt -- --check
• RUSTDOCFLAGS="-D warnings" cargo doc --all-features --document-private-items clean

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Added support for memo-sized bundles beyond the default configuration.
  • Expanded note encryption and bundle handling to work with different memo formats.

• Bug Fixes

  • Added safeguards to prevent invalid memo sizes from causing runtime failures.
  • Improved bundle commitment and transaction hash consistency checks.

• Tests

  • Added end-to-end coverage for memo handling and note recovery.
  • Added fixed golden-vector tests to verify stable encrypted output.

[coderabbitai]

📝 Walkthrough

Walkthrough

The MemoSize trait gains MEMO_SIZE and SIZE_CHECK associated constants for compile-time validation of memo/plaintext/ciphertext byte sizes. These checks are wired into note_encryption.rs and bundle/commitments.rs, and Bundle::from_parts for EffectsOnly is generalized over M. New tests validate golden vectors and Dash memo round-trips.

Changes

MemoSize Compile-Time Validation

Layer / File(s)	Summary
MemoSize trait constants and implementations src/memo.rs	Adds MEMO_SIZE and SIZE_CHECK associated constants to MemoSize, implements MEMO_SIZE=512 for ZcashMemo and MEMO_SIZE=36 for DashMemo, plus a test module validating both.
Wiring SIZE_CHECK into encryption and bundle constructor src/note_encryption.rs, src/bundle.rs	note_plaintext_bytes and split_plaintext_at_memo force evaluation of M::SIZE_CHECK; EffectsOnly's from_parts constructor becomes generic over M: MemoSize.
Txid hashing size check and golden vectors src/bundle/commitments.rs	hash_bundle_txid_data references M::SIZE_CHECK before slicing enc_ciphertext; new tests build deterministic bundles and assert fixed digest hex outputs for ZcashMemo and DashMemo.
Dash memo end-to-end tests tests/dash_memo.rs	New tests cover full/compact decryption and OVK recovery for a Dash-sized memo bundle, plus a golden-vector test for note encryption with exact ciphertext assertions.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Possibly related PRs

• dashpay/orchard#1: Introduces the generic MemoSize/M parameterization across Action/Bundle/note-encryption that this PR extends with MEMO_SIZE/SIZE_CHECK compile-time validation.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately captures the main change: compile-time MemoSize invariants and added DashMemo test coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/memo-size-invariants

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[coderabbitai]

🧹 Nitpick comments (1)

src/memo.rs (1)

47-69: 🎯 Functional Correctness | 🔵 Trivial | 💤 Low value

Document the SIZE_CHECK invariant for custom NoteBytes types. size_of only works as intended for implementations whose exposed byte length stays aligned with the associated size constants; a short note here would make that contract explicit for future MemoSize impls.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/memo.rs` around lines 47 - 69, Add a short doc note on
MemoSize::SIZE_CHECK explaining that custom NoteBytes/Memo implementations must
keep their exposed byte lengths exactly aligned with MEMO_SIZE,
COMPACT_NOTE_SIZE, and AEAD_TAG_SIZE. Mention that SIZE_CHECK enforces the
compile-time contract for MemoSize::Memo, NotePlaintextBytes, and
NoteCiphertextBytes so future impls understand the invariant and don’t rely on
runtime behavior.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@src/memo.rs`:
- Around line 47-69: Add a short doc note on MemoSize::SIZE_CHECK explaining
that custom NoteBytes/Memo implementations must keep their exposed byte lengths
exactly aligned with MEMO_SIZE, COMPACT_NOTE_SIZE, and AEAD_TAG_SIZE. Mention
that SIZE_CHECK enforces the compile-time contract for MemoSize::Memo,
NotePlaintextBytes, and NoteCiphertextBytes so future impls understand the
invariant and don’t rely on runtime behavior.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6750635f-9971-4e66-bdc9-983b1e9a219b

📥 Commits

Reviewing files that changed from the base of the PR and between 81ca424 and 47e937f.

📒 Files selected for processing (5)

• src/bundle.rs
• src/bundle/commitments.rs
• src/memo.rs
• src/note_encryption.rs
• tests/dash_memo.rs