ci: gate test matrix behind a tiered check/smoke/tutorials stage

Introduce a tiered CI structure:
- Tier 1 (ubuntu-latest, ungated): make check, make test-smoke,
  make test-tutorials.
- Tier 2 (needs all of Tier 1): the 4 linux test pipelines and the
  macOS test run.

Move `check` off the macOS self-hosted runner to ubuntu-latest and drop
its repo gate so the gate also runs on forks (otherwise Tier 2 would be
skipped there). test-smoke/test-tutorials targets are added by a
separate change; until then those jobs fail and Tier 2 is skipped.

Author: guru-desh

.github/workflows/ci.yaml

@@ -22,10 +22,11 @@ concurrency:
 
 jobs:
 
+  # ── Tier 1: cheap correctness gate (ubuntu-latest, ungated). ──
+  # Tier 2 runs only if all three of these pass.
   check:
-    name: macOS / make check
-    if: github.repository == 'apple/coreai-optimization'
-    runs-on: [self-hosted, macos, tahoe, ARM64]
+    name: Linux / make check
+    runs-on: ubuntu-latest
     timeout-minutes: 30
     steps:
       - name: Check out repository
@@ -39,9 +40,38 @@ jobs:
       - name: Run `make check`
         run: make check
 
+  test-smoke:
+    name: Linux / make test-smoke
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Install uv
+        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+        with:
+          enable-cache: false
+      - name: Run `make test-smoke`
+        run: make test-smoke
+
+  test-tutorials:
+    name: Linux / make test-tutorials
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - name: Install uv
+        uses: astral-sh/setup-uv@fac544c07dec837d0ccb6301d7b5580bf5edae39 # v8.2.0
+        with:
+          enable-cache: false
+      - name: Run `make test-tutorials`
+        run: make test-tutorials
+
+  # ── Tier 2: full test matrix, gated on Tier 1. ──
   macos-tests:
     name: macOS / make test-highest-pytorch / markers=not-slow
-    needs: [check]
+    needs: [check, test-smoke, test-tutorials]
     if: github.repository == 'apple/coreai-optimization'
     runs-on: [self-hosted, macos, tahoe, ARM64]
     timeout-minutes: 60
@@ -63,6 +93,7 @@ jobs:
 
   linux-highest-slow:
     name: Linux / make test-highest-pytorch / markers=slow
+    needs: [check, test-smoke, test-tutorials]
     runs-on: ubuntu-latest
     timeout-minutes: 120
     steps:
@@ -83,6 +114,7 @@ jobs:
 
   linux-highest-fast:
     name: Linux / make test-highest-pytorch / markers=not-slow
+    needs: [check, test-smoke, test-tutorials]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -103,6 +135,7 @@ jobs:
 
   linux-lowest-slow:
     name: Linux / make test-lowest-pytorch / markers=slow
+    needs: [check, test-smoke, test-tutorials]
     runs-on: ubuntu-latest
     timeout-minutes: 120
     steps:
@@ -123,6 +156,7 @@ jobs:
 
   linux-lowest-fast:
     name: Linux / make test-lowest-pytorch / markers=not-slow
+    needs: [check, test-smoke, test-tutorials]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps: