feat(release): add semantic-release auto versioning

- Add .releaserc.json: conventional commits -> semantic version bump
  (major/minor/patch), updates Cargo.toml + Cargo.lock version, writes
  CHANGELOG.md, commits as 'chore(release): X.Y.Z [skip ci]'
- Add release.yml: runs semantic-release on main push, then triggers
  publish.yml via workflow_dispatch with the new tag
- Update publish.yml: add workflow_dispatch input (tag) so release
  workflow can trigger publish directly (GITHUB_TOKEN tag pushes do not
  trigger 'on: push: tags'); still supports manual tag pushes

.releaserc.json

@@ -0,0 +1,18 @@
+{
+  "branches": ["main"],
+  "plugins": [
+    "@semantic-release/commit-analyzer",
+    "@semantic-release/release-notes-generator",
+    "@semantic-release/changelog",
+    ["@semantic-release/exec", {
+      "prepareCmd": "sed -i 's/^version = \\\"[^\\\"]*\\\"/version = \\\"${nextRelease.version}\\\"/' Cargo.toml && cargo check --locked"
+    }],
+    ["@semantic-release/git", {
+      "assets": ["Cargo.toml", "Cargo.lock", "CHANGELOG.md"],
+      "message": "chore(release): ${nextRelease.version} [skip ci]\n\n${nextRelease.notes}"
+    }],
+    ["@semantic-release/github", {
+      "assets": []
+    }]
+  ]
+}