release: v0.1.0

First cut release. Snapshots ./src/ as ./versions/v0.1.0/ via
`just cut-version v0.1.0` (22 files: 21 chapter MDs + SUMMARY.md).
Deployed alongside `latest` at:

  https://swaits.github.io/meshcore-spec/v0.1.0/

Also:

- CHANGELOG.md adopts Keep a Changelog-style version sections.
  Future entries land under [Unreleased] and graduate to a version
  heading when cut. The existing date-stamped pass entries stay
  put as a historical record of the work captured in v0.1.0.
- README.md fixes a stale "per vX.Y.Z tag" mention left over from
  the tag-based versioning model, and links the current release
  explicitly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

versions/v0.1.0/04-payload-ack.md

@@ -0,0 +1,99 @@
+# MeshCore Protocol Specification
+
+## Section 4: Payload — ACK
+
+### Overview
+
+The ACK (acknowledgment) payload is the simplest payload type. It consists of a
+single 4-byte CRC value used to confirm receipt of a previously sent message.
+The CRC is a truncated SHA-256 over the original TXT_MSG plaintext prefix
+concatenated with the sender's public key.
+
+### Payload Type
+
+Header payload type field: `0x03` (ACK)
+
+### Wire Format
+
+```
+ 0                   1                   2                   3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|                      ACK CRC (uint32_le)                      |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+```
+
+### Fields
+
+| Field | Offset | Size | Type | Description |
+|-------|--------|------|------|-------------|
+| ack_crc | 0 | 4 bytes | uint32_le | CRC identifying the acknowledged message |
+
+### ACK CRC Computation
+
+The 4-byte ACK CRC is computed as the first 4 bytes of a SHA-256 digest over
+the acknowledged TXT_MSG's plaintext prefix followed by the **sender's**
+Ed25519 public key:
+
+```
+ack_crc = SHA-256( plaintext_prefix || sender_pub_key )[0..3]
+```
+
+Where `plaintext_prefix` is the TXT_MSG plaintext up to and including the
+message text, **excluding any trailing NUL terminator**:
+
+```
+plaintext_prefix = timestamp_le(4) || txt_type_attempt(1) || text
+```
+
+- `timestamp_le` and `txt_type_attempt` are defined in
+  [Section 6](06-payload-encrypted.md#plaintext-format-txt_msg).
+- `text` is the raw UTF-8 message bytes with no NUL terminator.
+- `sender_pub_key` is the full 32-byte Ed25519 public key of the node that
+  originated the TXT_MSG (i.e., the node expecting this ACK back).
+
+For TXT_TYPE_SIGNED_PLAIN messages, the 4-byte signature prefix is included:
+
+```
+plaintext_prefix = timestamp_le(4) || txt_type_attempt(1) || signature(4) || text
+ack_crc = SHA-256( plaintext_prefix || receiver_pub_key )[0..3]
+```
+
+Note the SIGNED_PLAIN variant hashes against the **receiver's** public key;
+see `BaseChatMesh::onPeerDataRecv()`.
+
+Because `txt_type_attempt` includes the 2-bit `attempt` counter, each retry
+with an incremented `attempt` produces a different ACK CRC. This lets the
+sender attribute each received ACK to a specific transmission attempt. See
+[Section 6: Reliable DM Delivery](06-payload-encrypted.md#reliable-dm-delivery).
+
+### Encoding
+
+1. Compute the 4-byte CRC value for the message being acknowledged (above).
+2. Write the CRC as a little-endian 32-bit unsigned integer.
+3. Set payload_len to 4.
+
+### Decoding
+
+1. Read 4 bytes from the payload as a little-endian uint32.
+2. If fewer than 4 bytes are available, the packet is INVALID.
+
+### Constraints
+
+- The payload MUST be exactly 4 bytes.
+- Implementations SHOULD log or discard ACK packets with payload_len < 4.
+
+### Cross-References
+
+- [Section 1: Wire Format](01-wire-format.md) — Packet framing
+- [Section 11: Multipart](11-payload-multipart.md) — Multipart ACK encoding
+- Test vectors: `corpus/payloads/ack/`
+
+### Reference Implementation
+
+- `Mesh::createAck()` in `src/Mesh.cpp` — Packet construction
+- `BaseChatMesh::composeMsgPacket()` in `src/helpers/BaseChatMesh.cpp` —
+  Sender-side expected-ACK computation (`expected_ack`)
+- `BaseChatMesh::onPeerDataRecv()` in `src/helpers/BaseChatMesh.cpp` —
+  Receiver-side ACK CRC computation (`ack_hash`)
+- ACK reception in `Mesh::onRecvPacket()`, case `PAYLOAD_TYPE_ACK`