fix(dwarf): non-empty comp_dir so ld keeps the debug map (issue 0058)

A source path with no directory component (`sx build main.sx` from the
project dir — what the chess app does) made `diFileFor` emit a `DIFile`
with an empty `directory:`, so the compile unit's `DW_AT_comp_dir` was
"". Apple's ld then silently drops the *entire* object's debug map (0
N_OSO) and the binary is undebuggable — lldb resolves no sx source.
Builds whose path had any directory (`.sx-tmp/x.sx`, `examples/x.sx`)
were unaffected, which is why small repros + the stepping smoke passed
and only the bundled chess app hit it.

Fix: diFileFor falls back to "." (and "/" for a root-level file) when
the path has no directory component, so comp_dir is never empty.

Verified: chess (`sx build --target macos --emit-obj main.sx`) now
links with OSO=1 and lldb resolves `frame at main.sx:82:8`. Regression
guard added to the DWARF unit test (asserts `DIFile(... directory: ".")`
for a bare filename). Gates: zig build, zig build test, run_examples.sh
-> 291 passed, debug-stepping smoke ok.

src/ir/emit_llvm.zig

@@ -394,11 +394,15 @@ pub const LLVMEmitter = struct {
     }
 
     /// The `DIFile` for `path`, created once and cached. Splits the path
-    /// into basename + directory as DWARF expects.
+    /// into basename + directory as DWARF expects. The directory MUST be
+    /// non-empty: an empty `DW_AT_comp_dir` makes Apple's `ld` silently drop
+    /// the whole object's debug map (no `N_OSO`), so a binary built from a
+    /// bare filename (e.g. `sx build main.sx`) becomes undebuggable. Fall back
+    /// to "." when the path has no directory component.
     fn diFileFor(self: *LLVMEmitter, path: []const u8) c.LLVMMetadataRef {
         if (self.di_files.get(path)) |f| return f;
         const slash = std.mem.lastIndexOfScalar(u8, path, '/');
-        const dir = if (slash) |s| path[0..s] else "";
+        const dir = if (slash) |s| (if (s == 0) "/" else path[0..s]) else ".";
         const base = if (slash) |s| path[s + 1 ..] else path;
         const f = c.LLVMDIBuilderCreateFile(self.di_builder, base.ptr, base.len, dir.ptr, dir.len);
         self.di_files.put(path, f) catch {};