feat(C2): unit-first JIT symbol resolution — program-owned dylibs beat process images

runJITFromObject now takes priority dylibs (the #import c unit's
linked objects first, then #library deps in declaration order) and
attaches a per-path search generator for each AHEAD of the
process-wide fallback, so a vendored symbol can never lose to a
same-named export of an image the host process happens to carry
(libz via LLVM, libsqlite3 via CoreServices). loadLibrary reports
the name dlopen succeeded on; the c-import handle records its dylib
path; temp link inputs are per-pid so concurrent runs can't clobber
each other. Flips the C0.3 shadowing pin to from_unit: true.

src/c_import.zig

@@ -80,6 +80,9 @@ pub fn cSourceCacheKey(
 /// Handle returned from loadCObjectsForJIT — caller must call unload() after JIT.
 pub const CImportHandle = struct {
     dylib_handle: ?*anyopaque = null,
+    /// Where the unit's linked dylib lives for THIS run; the JIT adds it
+    /// as a priority symbol-search target ahead of the process images.
+    dylib_path: ?[:0]const u8 = null,
     temp_paths: []const []const u8 = &.{},
     allocator: std.mem.Allocator,
 
@@ -415,10 +418,12 @@ pub fn loadCObjectsForJIT(
 
     var temp_paths = std.ArrayList([]const u8).empty;
 
-    // Write each .o buffer to a temp file
+    // Write each .o buffer to a temp file (per-pid names: concurrent
+    // `sx run` processes must not clobber each other's link inputs)
+    const pid = std.c.getpid();
     var obj_paths = std.ArrayList([]const u8).empty;
     for (obj_bufs, 0..) |buf, i| {
-        const path = try std.fmt.allocPrint(allocator, "/tmp/sx_c_{d}.o", .{i});
+        const path = try std.fmt.allocPrint(allocator, "/tmp/sx_c_{d}_{d}.o", .{ pid, i });
         const start = c.LLVMGetBufferStart(buf);
         const size = c.LLVMGetBufferSize(buf);
         const data = @as([*]const u8, @ptrCast(start))[0..size];
@@ -432,8 +437,8 @@ pub fn loadCObjectsForJIT(
     }
 
     // Link into a shared library
-    const dylib_path = "/tmp/sx_c_import.dylib";
-    try temp_paths.append(allocator, try allocator.dupe(u8, dylib_path));
+    const dylib_path = try std.fmt.allocPrintSentinel(allocator, "/tmp/sx_c_import_{d}.dylib", .{pid}, 0);
+    try temp_paths.append(allocator, dylib_path);
 
     var argv = std.ArrayList([]const u8).empty;
     try argv.append(allocator, "cc");
@@ -477,6 +482,7 @@ pub fn loadCObjectsForJIT(
 
     return .{
         .dylib_handle = handle,
+        .dylib_path = dylib_path,
         .temp_paths = try temp_paths.toOwnedSlice(allocator),
         .allocator = allocator,
     };