ci: workflow_dispatch is a real dry run

- manual runs skip the tag guard (there is no tag on a dispatch, so
  GITHUB_REF_NAME is the branch and the guard always failed) and skip
  publishing
- a dispatch now builds, verifies the digest, smoke-tests the
  extracted toolchain and reports the glibc floor, then stops
- replaces the throwaway-tag rehearsal in the checklist: no tag to
  delete, no draft release to clean up

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/release.yml

@@ -6,7 +6,9 @@
 name: release
 
 # Fires only on a version tag, so nothing is published by an ordinary
-# push. `workflow_dispatch` is the manual escape hatch for a re-run.
+# push. `workflow_dispatch` is a DRY RUN: it builds, verifies and reports
+# the glibc floor, but skips the tag guard (there is no tag) and skips
+# publishing. Use it to rehearse before tagging anything.
 on:
   push:
     tags:
@@ -46,6 +48,7 @@ jobs:
       # nobody can install. mkdist.sh already guards VERSION against the
       # binaries; this guards the tag against VERSION.
       - name: Tag must match VERSION
+        if: github.event_name == 'push'
         run: |
           tag="${GITHUB_REF_NAME#v}"
           ver="$(cat VERSION)"
@@ -107,7 +110,9 @@ jobs:
 
       # gh is preinstalled on GitHub runners and reads GH_TOKEN from the
       # environment, so there is no `gh auth login` anywhere in this file.
+      # Skipped on workflow_dispatch: a dry run must never publish.
       - name: Publish
+        if: github.event_name == 'push'
         env:
           GH_TOKEN: ${{ github.token }}
         run: |

docs/guides/releasing.md

@@ -52,18 +52,16 @@ ignored by this workflow.
     publish step later fails with 403, come back and select
     "Read and write permissions".
 
- 5  REHEARSE before a real tag. Add --draft to the gh release create
-    line in .github/workflows/release.yml, commit, push.
+ 5  REHEARSE with a dry run — no tag, no publish, no cleanup.
+    Actions tab -> "release" -> "Run workflow" -> master.
+    A workflow_dispatch run skips the tag guard and the publish step,
+    so it builds, verifies, smoke-tests the extracted tarball and
+    reports the glibc floor, and stops there.
 
- 6  Fire it with a throwaway tag:
-      git tag -a v0.0.0-test -m "pipeline rehearsal"
-      git push origin v0.0.0-test
-    Note: the tag guard compares the tag to VERSION, so this run is
-    EXPECTED to fail at step "Tag must match VERSION". That is the
-    cheapest proof the guard works. To rehearse the whole job, set
-    VERSION to 0.0.0-test on a scratch branch and tag that instead.
+ 6  (nothing to undo — a dry run creates no tag and no release)
 
- 7  Watch it: Actions tab, or `gh run watch` once gh is authenticated.
+ 7  Watch it: the Actions tab, or `gh run watch` once gh is
+    authenticated.
 
  8  Read the "Report the glibc floor" step. It prints what the
     RUNNER-built binaries actually require. Expect 2.35-ish from