md-codec v0.9.0

[0.9.0] — 2026-04-29

Closes three mk1-surfaced FOLLOWUPS items:
chunk-set-id-rename,
md-path-dictionary-0x16-gap, and
path-dictionary-mirror-stewardship. Wire format
unchanged for the rename portion; wire-additive (0x16) for the new
testnet BIP 48 P2SH-P2WSH path indicator. Test-vector corpora regenerate
because GENERATOR_FAMILY rolls "md-codec 0.8" → "md-codec 0.9" and
because expected_error_variant strings rename in lockstep with code.

Why a rename, again?

v0.8.0 (2026-04-28) renamed WalletId → PolicyId to align with BIP 388's
policy-template framing (Tier-3 = "the policy"). That rename mechanically
renamed the chunk-header 20-bit field ChunkWalletId → ChunkPolicyId and
two error variants (WalletIdMismatch → PolicyIdMismatch,
ReservedWalletIdBitsSet → ReservedPolicyIdBitsSet) along with it. On
review for the sibling Mnemonic Key (mk1) BIP submission, this turned out
to be miscategorized: those names belong to the chunk-header sub-domain
and identify a chunk-set assembly — not a Policy ID, not a Wallet
Instance ID. v0.9.0 corrects the chunk-header sub-domain to
ChunkSetId/ChunkSetIdMismatch/ReservedChunkSetIdBitsSet. v0.8's
PolicyId and WalletInstanceId are stable and unchanged. We expect
this to be the last identifier rename in this family.

Changed (chunk-header field renames)

• ChunkPolicyId → ChunkSetId
• PolicyIdSeed → ChunkSetIdSeed
• EncodeOptions::policy_id_seed → EncodeOptions::chunk_set_id_seed
• EncodeOptions::with_policy_id_seed(seed) →
  EncodeOptions::with_chunk_set_id_seed(seed) (if/when the builder is
  used; check call sites)
• Error::PolicyIdMismatch { expected, got } →
  Error::ChunkSetIdMismatch { expected, got }
• Error::ReservedPolicyIdBitsSet → Error::ReservedChunkSetIdBitsSet
• ChunkHeader::Chunked.policy_id field → chunk_set_id
• Chunk.policy_id field → chunk_set_id
• Verifications.policy_id_consistent field → chunk_set_id_consistent
• BIP §"Wallet identifier" → §"Chunk-set identifier" with a v0.8→v0.9
  naming-note explaining the correction
• Test helpers: test_wallet_id, expected_wallet_id,
  chunked_round_trip_max_wallet_id, wid_a/wid_b →
  test_chunk_set_id, expected_chunk_set_id,
  chunked_round_trip_max_chunk_set_id, csid_a/csid_b

PolicyId, PolicyIdWords, WalletInstanceId,
compute_policy_id_for_policy, compute_wallet_instance_id,
MdBackup::policy_id() are intentionally unchanged.

Added

• Path-dictionary indicator 0x16 = m/48'/1'/0'/1' (BIP 48 testnet
  P2SH-P2WSH; mirror of mainnet 0x06). Wire-additive — existing
  decoders rejected 0x16 as an unknown indicator.
• Corpus vector t1_sh_wsh_testnet_0x16 exercising the new dictionary
  entry via EncodeOptions::with_shared_path("m/48'/1'/0'/1'").
• design/RELEASE_PROCESS.md documenting the md1↔mk1 path-dictionary
  lockstep release invariant and standard 16-step release checklist.
• BIP §"Path dictionary" trailing "Cross-format inheritance" paragraph.

Wire format

• Unchanged for the rename portion (chunk-set identifier is the same
  20-bit field, just spelled differently in code, prose, and JSON).
• Additive for 0x16 (forward-only — old encodings remain valid;
  encodings that select the testnet m/48'/1'/0'/1' path now serialize
  as a single 0x16 byte rather than the explicit-path fallback).
• Test-vector corpus JSON files (v0.1.json and v0.2.json)
  regenerated to absorb the renamed expected_error_variant strings,
  the family-token roll to "md-codec 0.9", and (for v0.2) the new T1
  vector. Both SHA pins update.

FOLLOWUPS closed

• chunk-set-id-rename (mk1-surfaced — hard precondition for mk1's BIP
  submission)
• md-path-dictionary-0x16-gap (mk1-surfaced)
• path-dictionary-mirror-stewardship (mk1-surfaced)

md-per-at-N-path-tag-allocation (the fourth mk1-surfaced item) remains
open and is deferred to v1+ pending per-cosigner-path scheduling.