test(C1.1): cSourceCacheKey — content key for compiled #source units

Source bytes, declared-header CONTENT (header edits invalidate),
defines/flags/include dirs in order, LLVM version, and target
triple/sysroot all participate; section tags keep equal strings in
different roles distinct. Pure function + variance property tests;
nothing consumes it yet.

src/c_import.test.zig

@@ -0,0 +1,68 @@
+const std = @import("std");
+const c_import = @import("c_import.zig");
+
+const SRC = "int f(void) { return 1; }";
+const HDR = "int f(void);";
+const VER = "19.1.7";
+
+const none: []const []const u8 = &.{};
+
+fn baseKey() u64 {
+    return c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+}
+
+test "cSourceCacheKey: stable when nothing changes" {
+    try std.testing.expectEqual(baseKey(), baseKey());
+}
+
+test "cSourceCacheKey: source bytes vary the key" {
+    const other = c_import.cSourceCacheKey("int f(void) { return 2; }", &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: declared header content varies the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{"int f(void); int g(void);"}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: defines vary the key (value and order)" {
+    const v2 = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=2"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != v2);
+
+    const ab = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{ "A=1", "B=1" }, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    const ba = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{ "B=1", "A=1" }, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(ab != ba);
+}
+
+test "cSourceCacheKey: flags vary the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O3"}, &.{"inc"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: a define is not a flag (same string, different role)" {
+    const as_define = c_import.cSourceCacheKey(SRC, none, &.{"X"}, none, none, VER, null, null);
+    const as_flag = c_import.cSourceCacheKey(SRC, none, none, &.{"X"}, none, VER, null, null);
+    try std.testing.expect(as_define != as_flag);
+}
+
+test "cSourceCacheKey: include dirs vary the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"other"}, VER, "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: llvm version varies the key" {
+    const other = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, "20.0.0", "arm64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other);
+}
+
+test "cSourceCacheKey: triple and sysroot vary the key; absent is not empty" {
+    const other_triple = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "x86_64-apple-darwin", "/sdk");
+    try std.testing.expect(baseKey() != other_triple);
+
+    const other_sysroot = c_import.cSourceCacheKey(SRC, &.{HDR}, &.{"A=1"}, &.{"-O2"}, &.{"inc"}, VER, "arm64-apple-darwin", "/ndk");
+    try std.testing.expect(baseKey() != other_sysroot);
+
+    const absent = c_import.cSourceCacheKey(SRC, none, none, none, none, VER, null, null);
+    const empty = c_import.cSourceCacheKey(SRC, none, none, none, none, VER, "", "");
+    try std.testing.expect(absent != empty);
+}