Merge the loader branch, key the build by the libnode pack, and pick musl packs

.github/workflows/ci.yml

@@ -52,9 +52,13 @@ jobs:
           const stackBase = pr?.stack?.base?.ref;
           const prBase = pr?.base?.ref;
           // Position 1 can already be merged; compare the current base branches.
+          // A PR into the default branch is the lowest unmerged layer of any
+          // stack, whatever the stack metadata names as its base.
+          const defaultBranch = event.repository?.default_branch;
           const upperLayer = process.env.GITHUB_EVENT_NAME === 'pull_request' &&
             sameRepository && trustedAuthor && typeof stackBase === 'string' && stackBase.length > 0 &&
-            typeof prBase === 'string' && prBase.length > 0 && stackBase !== prBase;
+            typeof prBase === 'string' && prBase.length > 0 && stackBase !== prBase &&
+            prBase !== defaultBranch;
           const full = !upperLayer;
           appendFileSync(process.env.GITHUB_OUTPUT, `full=${full}\n`);
           console.log(full ? 'Run full CI' : 'Defer full CI until this PR becomes the lowest unmerged layer');

.github/workflows/libnode-packs.yml

@@ -32,7 +32,9 @@ on:
       - ".github/workflows/libnode-packs.yml"
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  # A fixed name: under workflow_call, github.workflow is the caller's, and
+  # sharing the caller's group would cancel the run as a deadlock.
+  group: libnode-packs-${{ github.ref }}
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 permissions: