refactor(ffi-linkage): Phase 9.1a — rename collision-free linkage identifiers

Mechanical src/ rename of the linkage-family identifiers whose extern_* target is
collision-free: callForeign→callExtern, marshalForeignArg→marshalExternArg,
dedupeForeignSymbol→dedupeExternSymbol, foreign_name_map→extern_name_map,
is_foreign_c_api→is_extern_c_api. Snapshot-neutral (internal only); suite green
(646 corpus / 444 unit, 0 failed).

Deferred (need per-site analysis — target name already exists): is_foreign↔is_extern
(38 existing), foreign_lib/foreign_name↔extern_lib/extern_name (15/16 existing),
foreign_expr (still built by c_import.zig auto-synthesis). Runtime-class family
(ForeignClassDecl etc. → Runtime*, Decision 5) is Phase 9.2.

src/ir/interp.zig

@@ -405,7 +405,7 @@ pub const Interpreter = struct {
     /// underlying address. The returned `usize` is only valid for the
     /// duration of this call — caller-allocated buffers are tracked in
     /// `tmp` so they get freed once the call returns.
-    fn marshalForeignArg(self: *Interpreter, v: Value, tmp: *std.ArrayList([]u8)) !usize {
+    fn marshalExternArg(self: *Interpreter, v: Value, tmp: *std.ArrayList([]u8)) !usize {
         return switch (v) {
             .int => |i| @bitCast(i),
             .boolean => |b| @intFromBool(b),
@@ -488,7 +488,7 @@ pub const Interpreter = struct {
         }
     }
 
-    fn callForeign(self: *Interpreter, func: *const inst_mod.Function, args: []const Value) InterpError!Value {
+    fn callExtern(self: *Interpreter, func: *const inst_mod.Function, args: []const Value) InterpError!Value {
         const name = self.module.types.getString(func.name);
 
         // A foreign call may not return (e.g. `process.exit` → `_exit`), which
@@ -513,7 +513,7 @@ pub const Interpreter = struct {
             tmp.deinit(self.alloc);
         }
         for (args, 0..) |a, i| {
-            packed_args[i] = self.marshalForeignArg(a, &tmp) catch return error.TypeError;
+            packed_args[i] = self.marshalExternArg(a, &tmp) catch return error.TypeError;
         }
         const argv = packed_args[0..args.len];
 
@@ -569,7 +569,7 @@ pub const Interpreter = struct {
             // Dispatch to host libc via dlsym. Lets `#run` (and the
             // post-link bundler) call ordinary foreign symbols like
             // `puts`, `getenv`, `posix_spawn`, etc.
-            return self.callForeign(func, args);
+            return self.callExtern(func, args);
         }
 
         // Track the sx call chain for `trace.print_interpreter_frames()`.