test(e2e): assert compile_commands.json is generated on all platforms

Add a dedicated e2e (no requires → runs Linux/macOS/Windows) that builds a
minimal `mcpp new` project and verifies the Clang compilation database is
emitted at the project root: file exists + non-empty, top-level JSON array,
required keys (directory/file + command|arguments), an entry for src/main.cpp,
and a deeper python json parse where available.

CDB generation is unconditional in the ninja backend (write_compile_commands),
so this guards the IDE/clangd contract directly. Existing CDB tests cover
specifics (47 prebuilt-module-path abs, 59 std flag, gcc-only); this is the
plain cross-platform existence+validity guarantee. Verified: 47 already passes
on Windows/macOS CI, confirming the build path works there.

Author: Sunrisepeak

tests/e2e/76_compile_commands_generated.sh

@@ -0,0 +1,58 @@
+#!/usr/bin/env bash
+# requires:
+# 76_compile_commands_generated.sh — `mcpp build` of a minimal project must
+# emit a valid Clang compilation database (compile_commands.json) at the
+# project root, on EVERY platform (Linux / macOS / Windows). IDE/clangd
+# integration depends on it, and generation is unconditional in the ninja
+# backend, so this guards the contract directly rather than as a side effect
+# of the more specific CDB tests (47 prebuilt-module-path, 59 std flag).
+#
+# No `requires:` capability → runs on all three CI platforms.
+set -e
+
+TMP=$(mktemp -d)
+trap "rm -rf $TMP" EXIT
+
+cd "$TMP"
+"$MCPP" new app > /dev/null
+cd app
+"$MCPP" build > /dev/null
+
+cdb=compile_commands.json
+[[ -f "$cdb" ]] || { echo "FAIL: no $cdb generated at project root"; exit 1; }
+[[ -s "$cdb" ]] || { echo "FAIL: $cdb is empty"; exit 1; }
+
+# Top-level must be a JSON array.
+first="$(head -c 1 "$cdb")"
+[[ "$first" == "[" ]] || {
+    echo "FAIL: $cdb does not start with '[' (got '$first') — not a JSON array"
+    cat "$cdb"; exit 1
+}
+
+# CDB-required keys + the clang command form (command OR arguments).
+for key in '"directory"' '"file"'; do
+    grep -q "$key" "$cdb" || { echo "FAIL: $cdb missing $key"; cat "$cdb"; exit 1; }
+done
+grep -qE '"command"|"arguments"' "$cdb" || {
+    echo "FAIL: $cdb has neither \"command\" nor \"arguments\""; cat "$cdb"; exit 1
+}
+
+# The minimal project's source (src/main.cpp) must have an entry.
+grep -q 'main\.cpp' "$cdb" || { echo "FAIL: $cdb has no entry for src/main.cpp"; cat "$cdb"; exit 1; }
+
+# Deeper structural validation when a JSON parser is available (GitHub-hosted
+# runners ship python3). Skips cleanly where it isn't, keeping the grep checks
+# above as the portable baseline.
+if command -v python3 >/dev/null 2>&1; then
+    python3 - "$cdb" <<'PY' || exit 1
+import json, sys
+d = json.load(open(sys.argv[1], encoding="utf-8"))
+assert isinstance(d, list) and d, "CDB must be a non-empty JSON array"
+for e in d:
+    assert "file" in e and "directory" in e, "entry missing file/directory: %r" % e
+    assert ("command" in e) or ("arguments" in e), "entry missing command/arguments: %r" % e
+print("  json validation OK (%d entries)" % len(d))
+PY
+fi
+
+echo "OK"