repro: test conservative --target-cpu/--target-features as fix

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: sbrunk

.github/workflows/linux-repro.yml

@@ -47,34 +47,34 @@ jobs:
             grep -qw "$f" /proc/cpuinfo && echo "$f: yes" || echo "$f: NO"
           done
 
-      - name: Build + run repro
+      - name: Effective host target
+        run: |
+          source $HOME/.bash_profile
+          pixi run mojo build --print-effective-target /dev/stdin <<<'def main(): pass' 2>&1 | grep -iE "cpu|feature|avx512|target" | head || true
+
+      - name: Build + run repro (host vs conservative target)
         run: |
           source $HOME/.bash_profile
-          sudo apt-get update -qq && sudo apt-get install -y -qq gdb >/dev/null 2>&1 || true
           cat > repro.mojo <<'EOF'
           from duckdb import *
 
           def main() raises:
-              print("CHK 1: before libduckdb init")
-              ref lib = DuckDB().libduckdb()
-              print("CHK 2: version =", String(unsafe_from_utf8_ptr=lib.duckdb_library_version()))
               var con = DuckDB.connect(":memory:")
-              print("CHK 3: connected")
               var v = con.execute("SELECT 42 AS x").fetch_chunk().get[Int32](col=0, row=0)
-              print("CHK 4: fetched value =", v)
+              print("fetched value =", v)
           EOF
-          pixi run mojo build --debug-level=full repro.mojo -o ./repro_dbg
 
-          echo "============ run (debug AOT) ============"
-          if ./repro_dbg; then
-            echo "RESULT shard ${{ matrix.shard }}: OK"
-          else
-            code=$?
-            echo "RESULT shard ${{ matrix.shard }}: CRASH (exit $code, signal $((code-128)))"
-            echo "============ gdb backtrace ============"
-            gdb -q --batch \
-              -ex 'run' \
-              -ex 'echo \n=== backtrace ===\n' -ex 'bt' \
-              -ex 'echo \n=== faulting instruction ===\n' -ex 'x/i $pc' \
-              ./repro_dbg 2>&1 | tail -45 || true
-          fi
+          run_cfg () {
+            local label="$1"; shift
+            if pixi run mojo build "$@" repro.mojo -o ./repro_bin 2>/dev/null && ./repro_bin >/dev/null 2>&1; then
+              echo "RESULT shard ${{ matrix.shard }} [$label]: OK"
+            else
+              echo "RESULT shard ${{ matrix.shard }} [$label]: CRASH (signal $(( $? - 128 )))"
+            fi
+          }
+
+          echo "avx512f on this runner: $(grep -qw avx512f /proc/cpuinfo && echo yes || echo NO)"
+          run_cfg "host-default"
+          run_cfg "x86-64-v3"  --target-cpu x86-64-v3
+          run_cfg "x86-64-v2"  --target-cpu x86-64-v2
+          run_cfg "no-avx512"  --target-features=-avx512f,-avx512vl,-avx512bw,-avx512cd,-avx512dq