sema: fix cross-module constant resolution timeout

Guard evalCrossModuleDeclValue mini-sema to prevent recursive resolution
and expensive file I/O:
- Only attempt cross-module resolution in comptime blocks
- Do not propagate source_dir to mini-sema to prevent infinite recursion
  (e.g. common.zig -> builtin.abi -> ...)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

stage0/sema.c

@@ -5808,10 +5808,13 @@ static AirInstRef evalCrossModuleDeclValue(
 
     // Create a mini-sema with the imported module's ZIR.
     // Shares the InternPool so IP indices are valid across both semas.
+    // Do NOT set source_dir: this prevents recursive cross-module
+    // resolution (e.g. common.zig -> builtin.abi) from causing
+    // infinite recursion or expensive file I/O.
     Sema mini_sema;
     semaInit(&mini_sema, parent_sema->ip, *import_zir);
-    mini_sema.source_dir = parent_sema->source_dir;
-    mini_sema.module_root = parent_sema->module_root;
+    mini_sema.source_dir = NULL;
+    mini_sema.module_root = NULL;
 
     SemaBlock ct_block;
     semaBlockInit(&ct_block, &mini_sema, NULL);
@@ -5945,8 +5948,10 @@ static AirInstRef zirFieldValComptime(
     // evaluate its value body using a mini-sema.
     // This handles patterns like: common.want_aeabi where common is
     // @import("./common.zig") and want_aeabi is a pub const bool.
+    // Only attempt in comptime context to avoid expensive file I/O
+    // on every runtime field_val instruction.
     if (obj_ip == IP_INDEX_VOID_VALUE && obj_ref >= ZIR_REF_START_INDEX
-        && sema->source_dir) {
+        && sema->source_dir && block->is_comptime) {
         uint32_t obj_inst = obj_ref - ZIR_REF_START_INDEX;
         ZirInstTag obj_tag = sema->code.inst_tags[obj_inst];
         if (obj_tag == ZIR_INST_DECL_VAL || obj_tag == ZIR_INST_DECL_REF) {