stdlib: relocate modules under library/

- examples/modules/ -> library/modules/ (top-level, no more
  symlink hacks in consumer projects)
- compiler discovers stdlib via _NSGetExecutablePath / readlink
  /proc/self/exe; searches dev layout (../../library), install
  layout (../library), and alongside-binary fallback
- SX_STDLIB_PATH env var overrides for tests / dev convenience
- SX_DEBUG_STDLIB env var dumps the discovery results
- build.zig installs library/ alongside the binary
- Compilation gains stdlib_paths field threaded through resolveImports
- 50 tests pass; consumer projects can now build from any cwd

src/core.zig

@@ -18,6 +18,7 @@ pub const Compilation = struct {
     source: [:0]const u8,
     diagnostics: errors.DiagnosticList,
     target_config: TargetConfig,
+    stdlib_paths: []const []const u8 = &.{},
 
     // Pipeline results
     root: ?*Node = null,
@@ -27,7 +28,7 @@ pub const Compilation = struct {
     sema_result: ?sema.SemaResult = null,
     ir_emitter: ?ir.LLVMEmitter = null,
 
-    pub fn init(allocator: std.mem.Allocator, io: std.Io, file_path: []const u8, source: [:0]const u8, target_config: TargetConfig) Compilation {
+    pub fn init(allocator: std.mem.Allocator, io: std.Io, file_path: []const u8, source: [:0]const u8, target_config: TargetConfig, stdlib_paths: []const []const u8) Compilation {
         return .{
             .allocator = allocator,
             .io = io,
@@ -37,6 +38,7 @@ pub const Compilation = struct {
             .import_sources = std.StringHashMap([:0]const u8).init(allocator),
             .module_scopes = std.StringHashMap(std.StringHashMap(void)).init(allocator),
             .target_config = target_config,
+            .stdlib_paths = stdlib_paths,
         };
     }
 
@@ -66,6 +68,7 @@ pub const Compilation = struct {
             &cache,
             &self.import_sources,
             &self.diagnostics,
+            self.stdlib_paths,
         ) catch return error.CompileError;
 
         // Preserve per-module visibility scopes for C import access checking