lang F1 6: generic-struct -> parameterized-protocol erasure (issue 0054 FIXED)

Two fixes, root-caused from xx Combined -> VL(s64) trapping:
- instantiateGenericStruct binds the template name to the concrete instance
  (tb.put(tmpl.name, id)), so an impl method self: *Combined resolves self.field
  to the instance (Combined__s64_s64), not the 0-field generic stub. This was a
  general pre-existing bug: self.x on ANY generic-struct impl method failed.
- createProtocolThunk monomorphizes the template method for a generic-struct
  instance (Combined.get -> Combined__s64_s64.get with the instance bindings),
  so the erasure vtable dispatches instead of hitting an unreachable thunk.

xx c on a generic Combined now dispatches correctly (examples/212 -> 99).
247 examples + unit green.

examples/212-generic-struct-protocol-erase.sx

@@ -0,0 +1,33 @@
+// Phase 6 / issue 0054 — generic-struct → parameterized-protocol erasure.
+//   - A generic-struct impl method `self: *Box` now resolves `self.field` to the
+//     concrete INSTANCE (the template name binds to the instance type), so
+//     `self.x` works (was "field not found on type 'Box'").
+//   - `xx c` erases a generic-struct instance (`Combined__s64_s64`) to a
+//     parameterized protocol (`VL(s64)`) via the generic
+//     `impl VL($R) for Combined($R, ..$Ts)`: the thunk monomorphizes the
+//     template method for the instance and dispatch works (was a trap).
+
+#import "modules/std.sx";
+
+VL :: protocol(T: Type) { get :: () -> T; }
+IntCell :: struct { v: s64; }
+impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+
+Combined :: struct($R: Type, ..$Ts: []Type) {
+    sources: (..VL(Ts));
+    value:   $R;
+}
+impl VL($R) for Combined($R, ..$Ts) { get :: (self: *Combined) -> $R => self.value; }
+
+make :: (..sources: VL) -> VL(s64) {
+    c : Combined(s64, ..sources.T) = ---;
+    c.value = 99;
+    c.sources = (..sources);
+    return xx c;                 // Combined__s64_s64 -> VL(s64)
+}
+
+main :: () -> s32 {
+    r := make(IntCell.{ v = 1 });
+    print("{}\n", r.get());      // 99 (dispatch through the erased Combined)
+    0;
+}

src/ir/lower.zig

@@ -11604,6 +11604,11 @@ pub const Lowering = struct {
         const id = if (table.findByName(name_id)) |existing| existing else table.intern(info);
         table.update(id, info);
 
+        // Bind the template name to this concrete instance so a method's
+        // `self: *Combined` (the template name) resolves to `*Combined__s64_s64`
+        // — otherwise `self.field` hits the 0-field generic stub.
+        tb.put(tmpl.name, id) catch {};
+
         // Store the type bindings and template name for method resolution
         const owned_mangled = self.alloc.dupe(u8, mangled_name) catch return id;
         self.struct_instance_bindings.put(owned_mangled, tb) catch {};
@@ -12879,8 +12884,21 @@ pub const Lowering = struct {
 
         // Ensure the concrete method is lowered
         const qualified = std.fmt.allocPrint(self.alloc, "{s}.{s}", .{ concrete_type_name, method.name }) catch method.name;
-        if (self.fn_ast_map.contains(qualified) and !self.lowered_functions.contains(qualified)) {
-            self.lazyLowerFunction(qualified);
+        if (!self.lowered_functions.contains(qualified)) {
+            if (self.fn_ast_map.contains(qualified)) {
+                self.lazyLowerFunction(qualified);
+            } else if (self.struct_instance_template.get(concrete_type_name)) |tmpl_name| {
+                // Generic-struct instance (`Combined__s64_s64`): the impl method
+                // is registered under the template name (`Combined.get`).
+                // Monomorphize it for this instance's bindings so the thunk has a
+                // concrete `Combined__s64_s64.get` to call.
+                const tmpl_qualified = std.fmt.allocPrint(self.alloc, "{s}.{s}", .{ tmpl_name, method.name }) catch method.name;
+                if (self.fn_ast_map.get(tmpl_qualified)) |fd| {
+                    if (self.struct_instance_bindings.getPtr(concrete_type_name)) |bindings| {
+                        self.monomorphizeFunction(fd, qualified, bindings);
+                    }
+                }
+            }
         }
 
         // Call the concrete method: ConcreteType.method(__sx_ctx?, ctx, args...).

tests/expected/212-generic-struct-protocol-erase.exit

@@ -0,0 +1 @@
+0

tests/expected/212-generic-struct-protocol-erase.txt

@@ -0,0 +1 @@
+99