lang F1 Phase 6: canonical heterogeneous map — $R inference through closure params

The full canonical `map` now compiles and runs (examples/213 → 42):

    map :: (mapper: Closure(..sources.T) -> $R, ..sources: VL) -> VL($R)

Final piece: infer a pack-fn's generic return `$R` from a closure-typed
prefix param's lowered return type.

- collectGenericNames descends into closure_type_expr (params + return),
  so `$R` in `Closure(..) -> $R` registers as a function type-param.
- matchTypeParam/extractTypeParam descend into closures: `$R` is extracted
  from the lowered mapper's closure `.ret`.
- lowerPackFnCall infers type-param bindings from the lowered prefix args,
  folds them into the mangle, and threads them into monomorphizePackFn,
  which installs self.type_bindings for return-type resolution + body
  lowering (`-> VL($R)` ⇒ VL(s64); `Combined($R, ..)` ⇒ Combined(s64, ..)).

s64-elimination follow-through:

- An unbound generic `$R` resolves to `.unresolved` in resolveTypeWithBindings
  rather than fabricating an empty-struct stub (`R{}`).
- Lambda return-type inference skips an `.unresolved` target-closure ret and
  infers from the body, so the concrete return drives `$R`.
- The `.unresolved` codegen tripwire then caught a latent bug: a generic-struct
  source impl (`impl VL($R) for Combined($R, ..$Ts)`) was declaring its template
  method `Combined.get` (`-> $R`) as a standalone IR function. Fixed: a
  generic-struct source registers methods as TEMPLATES only (findable in
  fn_ast_map for per-instance monomorphization via createProtocolThunk), never
  declareFunction'd.

Feature 1 (heterogeneous variadic packs) all six phases complete.
248 examples + all unit tests green.

examples/213-canonical-map.sx

@@ -0,0 +1,36 @@
+// Phase 6 — the canonical heterogeneous `map`, end to end. A pack-fn whose
+// return type `$R` is inferred from the mapper's closure return:
+//   - `mapper: Closure(..sources.T) -> $R` types the lambda's params from the
+//     projected pack element types, and its body (`a + b`) drives `$R`.
+//   - `$R` is inferred at the call site from the lowered mapper's closure ret,
+//     bound into the mono (`-> VL($R)` ⇒ `VL(s64)`, `Combined($R, ..)` ⇒
+//     `Combined(s64, ..)`), and folded into the mangle.
+//   - `(..sources)` materializes the pack into the `(..VL(Ts))` field (per-element
+//     erase) and `mapper(..sources.get)` projects+spreads; `xx c` erases the
+//     generic-struct instance to `VL(s64)` via the generic impl's monomorphized
+//     thunk.
+
+#import "modules/std.sx";
+
+VL :: protocol(T: Type) { get :: () -> T; }
+IntCell :: struct { v: s64; }
+impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+
+Combined :: struct($R: Type, ..$Ts: []Type) {
+    sources: (..VL(Ts));
+    value:   $R;
+}
+impl VL($R) for Combined($R, ..$Ts) { get :: (self: *Combined) -> $R => self.value; }
+
+map :: (mapper: Closure(..sources.T) -> $R, ..sources: VL) -> VL($R) {
+    c : Combined($R, ..sources.T) = ---;
+    c.sources = (..sources);
+    c.value = mapper(..sources.get);
+    return xx c;
+}
+
+main :: () -> s32 {
+    r := map((a, b) => a + b, IntCell.{ v = 40 }, IntCell.{ v = 2 });
+    print("{}\n", r.get());     // 42
+    0;
+}