spec: link corpus references to GitHub, version-aware

Every "Test vectors: \`corpus/...\`" mention in the spec is now a
markdown link pointing at the corresponding file or directory on
GitHub. Each version build resolves its links to the correct git
ref:

  - /latest/ -> https://github.com/swaits/meshcore-spec/tree/main/corpus/...
  - /v0.1.0/ -> https://github.com/swaits/meshcore-spec/tree/v0.1.0/corpus/...
  - future /vX.Y.Z/ -> /tree/vX.Y.Z/corpus/...

Two changes:

1. src/*.md (18 files): the inline-code corpus paths in each
   "Cross-References" section are now markdown links. Directories
   point at /tree/main/corpus/..., individual .json files point at
   /blob/main/corpus/.... CHANGELOG and other non-corpus main links
   are intentionally left pointing at main — the canonical history
   lives there regardless of which version you're reading.

2. justfile: `cut-version` now also runs a sed over the snapshot's
   *.md files replacing /tree/main/corpus -> /tree/<VERSION>/corpus
   and /blob/main/corpus -> /blob/<VERSION>/corpus. So a freshly-cut
   version is self-consistent.

versions/v0.1.0/* regenerated by re-running
`just cut-version v0.1.0` after the new src/ links + recipe sed
landed.

Tagging the release commit (`git tag v0.1.0`) is now REQUIRED — not
"optional" — because the rendered /v0.1.0/ pages reference
/tree/v0.1.0/corpus/... on GitHub. Without the tag the links 404.
cut-version's output now says so.

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

src/18-companion-protocol.md

@@ -89,7 +89,7 @@ implementations SHOULD request a larger MTU (up to 512 bytes).
 ### Cross-References
 
 - [Section 1: Wire Format](01-wire-format.md) — Underlying packet format
-- Test vectors: `corpus/companion/`
+- Test vectors: [`corpus/companion/`](https://github.com/swaits/meshcore-spec/tree/main/corpus/companion/)
 
 ### Reference Implementation