chore: housekeeping — rescue orphaned doc, ignore .DS_Store, adopt vnc-v0.3.0

Commits the offline-client architecture analysis doc that was sitting
untracked in docs/ — its own header already warns this exact thing
happened once before (~/vncmail-plus is a shared checkout; an earlier
untracked copy was lost to a concurrent branch switch). Confirmed the
hazard is still live: vnc/VNC-CHANGES.md itself was found deleted from
disk mid-edit by this session, by something else touching the checkout
concurrently, and had to be restored with `git checkout --` before this
commit. Committing on sight is the only defense against that, not a
process improvement for later.

Also:
- .DS_Store added to .gitignore (was untracked in docs/)
- introduces a VNC-side feature version, separate from package.json's
  upstream-tracking version (1.7.8, must stay that way per the fork's own
  rule 4 - bumping it would turn merging upstream releases into a diffing
  exercise). Retroactively bucketed at the milestone boundaries the commit
  history already has: v0.1.0 fork bootstrap, v0.2.0 S/MIME plugin
  audit+fixes, v0.3.0 the internal-CA foundation just landed. Tagged
  vnc-v0.3.0 on this commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -58,3 +58,6 @@ next-env.d.ts
 vnc/plugins/smime/node_modules/
 vnc/plugins/smime/dist/
 vnc/plugins/smime/smime-vnc.zip
+
+# macOS
+.DS_Store