Make the macOS Intel build leg blocking so darwin-amd64 always ships

The macos-15-intel leg had continue-on-error, so when that runner was unavailable the darwin-amd64 artifact silently never built and releases shipped with no Intel macOS binary. Remove continue-on-error so the leg is blocking - the binary ships or the build fails loudly. macos-15-intel is GitHub's supported Intel image through Aug 2027.

.github/workflows/_build.yml

@@ -39,11 +39,12 @@ jobs:
             cc: cc
             cxx: c++
     runs-on: ${{ matrix.os }}
-    # GitHub is sunsetting Intel-macOS runners, so the macos-15-intel leg is
-    # frequently cancelled before it can be allocated. Mark only that leg
-    # non-blocking so a scarce runner can't gate the aggregate build result
-    # (and thus skip smoke/soak in the dry-run). All other legs stay required.
-    continue-on-error: ${{ matrix.os == 'macos-15-intel' }}
+    # Every leg is BLOCKING — no continue-on-error. The darwin-amd64 binary
+    # built on macos-15-intel must ship with every release; if that runner is
+    # unavailable the build fails loudly rather than silently publishing a
+    # release with no Intel macOS binary (a user-reported gap). macos-15-intel
+    # is GitHub's supported Intel image through Aug 2027 (the last x86_64 macOS
+    # runner); revisit the Intel leg before that retirement.
     timeout-minutes: 15
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2

.github/workflows/dry-run.yml

@@ -58,9 +58,9 @@ jobs:
     uses: ./.github/workflows/_build.yml
 
   # ── Smoke test every binary ────────────────────────────────────
-  # Run unless builds were skipped or a build leg genuinely FAILED. A
-  # 'cancelled' aggregate caused only by the non-blocking macos-15-intel leg
-  # (scarce Intel-macOS runner) must NOT skip smoke — see _build.yml.
+  # Run unless builds were skipped or a build leg FAILED. Every build leg
+  # (including the macos-15-intel darwin-amd64 binary) is blocking now, so a
+  # failed/missing platform binary correctly stops smoke — see _build.yml.
   smoke:
     if: ${{ inputs.skip_builds != true && !cancelled() && needs.build.result != 'failure' && needs.build.result != 'skipped' }}
     needs: [build]