feat: HTTP registry, app/bench harnesses, and DWARF #line maps

Ship QUALITY_PLAN sessions 31–33: fetchable package index URLs, showcase
app and micro/nexus benchmarks, and debugger-friendly C codegen.

- Registry: BUX_REGISTRY accepts http(s) URLs (curl/wget → ~/.bux/cache)
- E.2: make test-apps smoke for nexus/boko/simpledb/jwt-pitbul
- E.5: benches/micro + C/Nim/Zig twins; make bench-nexus (wrk)
- E.4: HIR locs → #line .bux; default -O0 -g; --release -O2; make test-dwarf

tools/smoke_dwarf.sh

@@ -0,0 +1,88 @@
+#!/usr/bin/env bash
+# Smoke: DWARF / #line maps for debugger (E.4)
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+BUXC="$ROOT/buxc"
+
+if [[ ! -x "$BUXC" ]]; then
+  (cd "$ROOT" && make build >/dev/null)
+fi
+
+PKG="$ROOT/examples_pkg/hello"
+mkdir -p "$PKG/src"
+cp "$ROOT/examples/hello.bux" "$PKG/src/Main.bux"
+if [[ ! -f "$PKG/bux.toml" ]]; then
+  cat > "$PKG/bux.toml" <<'EOF'
+[Package]
+Name    = "hello"
+Version = "0.1.0"
+Type    = "bin"
+
+[Build]
+Output = "Bin"
+EOF
+fi
+
+echo "=== debug build (default -O0 -g + #line) ==="
+"$BUXC" build "$PKG"
+MAIN_C="$PKG/build/main.c"
+BIN="$PKG/build/hello"
+test -f "$MAIN_C"
+test -x "$BIN"
+
+# Expect #line pointing at .bux sources (user or stdlib)
+if ! grep -qE '^#line [0-9]+ ".*\.bux"' "$MAIN_C"; then
+  echo "error: no #line …\".bux\" directives in $MAIN_C" >&2
+  grep -n '#line' "$MAIN_C" | sed -n '1,5p' || true
+  exit 1
+fi
+echo "  #line maps present:"
+grep -E '^#line [0-9]+ ".*\.bux"' "$MAIN_C" | sed -n '1,5p' || true
+# User Main should map back to the package Main.bux
+if ! grep -qE '^#line [0-9]+ ".*Main\.bux"' "$MAIN_C"; then
+  echo "error: no #line for user Main.bux" >&2
+  exit 1
+fi
+echo "  user Main.bux #line present"
+
+# DWARF sections
+if command -v readelf >/dev/null 2>&1; then
+  if ! readelf -S "$BIN" | grep -q '\.debug_info'; then
+    echo "error: no .debug_info in $BIN" >&2
+    exit 1
+  fi
+  echo "  .debug_info present"
+  if readelf -p .debug_str "$BIN" 2>/dev/null | grep -q '\.bux'; then
+    echo "  .debug_str contains .bux paths"
+  else
+    echo "  (note: .debug_str may omit .bux; line tables still OK)"
+  fi
+fi
+
+# gdb: list Main
+if command -v gdb >/dev/null 2>&1; then
+  echo "=== gdb list Main ==="
+  set +e
+  gdb -batch -ex "file $BIN" -ex "list Main" 2>/dev/null | sed -n '1,20p'
+  set -e
+fi
+
+echo "=== release build (no #line, no -g required) ==="
+"$BUXC" build --release "$PKG"
+if grep -qE '^#line ' "$MAIN_C"; then
+  echo "error: release build still has #line" >&2
+  exit 1
+fi
+echo "  release: no #line (OK)"
+if command -v readelf >/dev/null 2>&1; then
+  if readelf -S "$BIN" | grep -q '\.debug_info'; then
+    echo "  (release still has debug sections — unexpected but non-fatal)"
+  else
+    echo "  release: no .debug_info (OK)"
+  fi
+fi
+
+# Rebuild debug so leftover state is debug-friendly for other tests
+"$BUXC" build "$PKG" >/dev/null
+
+echo "PASS: dwarf smoke (#line + .debug_info + --release)"