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;
+}