feat(versioning): snapshot corpus per spec version

A frozen spec is only meaningful if it's paired with the corpus
that was current at release time — otherwise a v0.1.0 reader
follows "see corpus/payloads/ack/" and lands on a file whose
test vectors may have changed since v0.1.0 was cut.

Three changes:

1. `cut-version` now copies BOTH src/ AND corpus/ into the
   snapshot directory. A release is the pair, not just the text.

2. The corpus URL rewriting in cut-version now retargets
   /tree/main/corpus -> /tree/main/versions/<v>/corpus (and the
   blob variant). This lets the link resolve on GitHub from main
   alone — no git tag required, since the frozen copy is a real
   path under main. Downgrades the "tag is REQUIRED" message from
   last commit back to "optionally tag the commit".

3. The frozen corpus's spec_ref fields get rewritten too: from
   `src/04-payload-ack.md` to `versions/<v>/04-payload-ack.md`,
   so a reader following a test vector lands on the spec text the
   vector was paired with at release time.

versions/v0.1.0/ regenerated from scratch via `just cut-version v0.1.0`
to pick up the corpus snapshot + the new sed patterns. The release
is now self-consistent: v0.1.0 spec links go to v0.1.0 corpus, which
links back to v0.1.0 spec.

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

versions/v0.1.0/corpus/payloads/group/grp-data.json

@@ -0,0 +1,38 @@
+{
+  "$schema": "../../schema/test-vector.schema.json",
+  "name": "Group Data Payload",
+  "spec_ref": "versions/v0.1.0/08-payload-group.md",
+  "protocol_version": 0,
+  "description": "Tests group datagram payload. Same wire format as GRP_TXT but payload type = 0x06.",
+  "vectors": [
+    {
+      "id": "grp-data-001",
+      "description": "Group datagram with known channel secret",
+      "type": "encode_decode",
+      "structured": {
+        "header": {
+          "version": 0,
+          "payload_type": "grp_data",
+          "route_type": "flood"
+        },
+        "path": {
+          "hash_size": 1,
+          "hash_count": 0,
+          "hashes": []
+        },
+        "payload": {
+          "channel_hash": "72",
+          "cipher_mac": "D184",
+          "ciphertext": "948C819389BA4ED7B1194A0FF8E62073"
+        }
+      },
+      "binary": "19 00 72 D184 948C819389BA4ED7B1194A0FF8E62073",
+      "crypto_context": {
+        "shared_secret": "202122232425262728292A2B2C2D2E2F303132333435363738393A3B3C3D3E3F",
+        "encryption_key": "202122232425262728292A2B2C2D2E2F",
+        "plaintext": "47726F75704D736721"
+      },
+      "notes": "Header 0x19 = GRP_DATA(6)+FLOOD(1). Same ciphertext as GRP_TXT since same key and plaintext."
+    }
+  ]
+}

versions/v0.1.0/corpus/payloads/group/grp-txt.json

@@ -0,0 +1,46 @@
+{
+  "$schema": "../../schema/test-vector.schema.json",
+  "name": "Group Text Payload",
+  "spec_ref": "versions/v0.1.0/08-payload-group.md",
+  "protocol_version": 0,
+  "description": "Tests group text message payload: channel_hash(1) + cipher_mac(2) + ciphertext.",
+  "vectors": [
+    {
+      "id": "grp-txt-001",
+      "description": "Group text message with known channel secret",
+      "type": "encode_decode",
+      "structured": {
+        "header": {
+          "version": 0,
+          "payload_type": "grp_txt",
+          "route_type": "flood"
+        },
+        "path": {
+          "hash_size": 1,
+          "hash_count": 0,
+          "hashes": []
+        },
+        "payload": {
+          "channel_hash": "72",
+          "cipher_mac": "D184",
+          "ciphertext": "948C819389BA4ED7B1194A0FF8E62073"
+        }
+      },
+      "binary": "15 00 72 D184 948C819389BA4ED7B1194A0FF8E62073",
+      "crypto_context": {
+        "shared_secret": "202122232425262728292A2B2C2D2E2F303132333435363738393A3B3C3D3E3F",
+        "encryption_key": "202122232425262728292A2B2C2D2E2F",
+        "plaintext": "47726F75704D736721"
+      },
+      "notes": "Channel hash = SHA256(channel_secret)[0] = 0x72. Plaintext 'GroupMsg!' (9 bytes). AES key = first 16 bytes of channel secret."
+    },
+    {
+      "id": "grp-txt-002",
+      "description": "Group text with tampered MAC must fail",
+      "type": "invalid",
+      "binary": "15 00 72 0000 948C819389BA4ED7B1194A0FF8E62073",
+      "expected_error": "mac_invalid",
+      "notes": "Same ciphertext as grp-txt-001 but MAC zeroed. Verification must fail."
+    }
+  ]
+}