fix(ir): poison type-fn binder on failed value-param bind (0083)

A failed value-param bind on a type-returning function (e.g.
`MakeC :: ($K: Count, $T: Type) -> Type { return [K]T; }` with
`a : MakeC(5_000_000_000, s64)`) emitted its correct range diagnostic
but then `instantiateTypeFunction` returned `null`, so
`resolveParameterizedWithBindings` fell through to an empty-struct
placeholder named after the function. The binding `a` got that
placeholder type, so a later `a.len` cascaded a bogus second error
`field 'len' not found on type 'MakeC'`.

The struct binder (`instantiateGenericStruct`) already returns
`.unresolved` here; the type-fn binder now matches it — a failed
value-param bind poisons to `.unresolved` instead of `null`, so the
caller propagates the diagnosed poison and the existing
`emitFieldError` suppression yields one clean diagnostic. Covers
every type-fn value-param failure mode: overflow via an aliased
constraint, a non-const arg, and an unknown type arg.

Regression: examples/1137-diagnostics-value-param-type-fn-no-cascade.sx

examples/1137-diagnostics-value-param-type-fn-no-cascade.sx

@@ -0,0 +1,27 @@
+// A FAILED value-param bind on a type-RETURNING FUNCTION must emit exactly its
+// own diagnostic and NOT cascade a bogus `field '…' not found on '<fn>'` when the
+// binding is later field-accessed. The type-fn binder must poison the binding to
+// `.unresolved` (the diagnosed-poison sentinel) — exactly like the struct binder —
+// so the downstream `.len` is suppressed, not reported as a second error.
+//
+// Regression (issue 0083): the type-fn path (`instantiateTypeFunction`) fell
+// through to an empty-struct placeholder named after the function on a failed
+// value-param bind, so `a.len` produced a second `field 'len' not found on
+// 'MakeC'` error. The struct binder already returned `.unresolved` here; the
+// type-fn binder now matches it. Three failure modes, three clean diagnostics,
+// zero field cascades:
+//   - value-param overflow via an aliased integer constraint (`$K: Count`),
+//   - a non-const value-param arg (`get()`),
+//   - an unknown TYPE arg (`NoSuchType`) — must still report the unknown type.
+#import "modules/std.sx";
+
+Count :: u32;
+MakeC :: ($K: Count, $T: Type) -> Type { return [K]T; }
+get  :: () -> u32 { return 4; }
+
+main :: () {
+    a : MakeC(5000000000, s64) = ---;
+    b : MakeC(get(), s64) = ---;
+    c : MakeC(3, NoSuchType) = ---;
+    print("unreachable {} {} {}\n", a.len, b.len, c.len);
+}

examples/expected/1137-diagnostics-value-param-type-fn-no-cascade.exit

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

examples/expected/1137-diagnostics-value-param-type-fn-no-cascade.stderr

@@ -0,0 +1,17 @@
+error: unknown type 'NoSuchType'
+  --> examples/1137-diagnostics-value-param-type-fn-no-cascade.sx:25:18
+   |
+25 |     c : MakeC(3, NoSuchType) = ---;
+   |                  ^^^^^^^^^^
+
+error: value 5000000000 does not fit in u32 parameter K
+  --> examples/1137-diagnostics-value-param-type-fn-no-cascade.sx:23:15
+   |
+23 |     a : MakeC(5000000000, s64) = ---;
+   |               ^^^^^^^^^^
+
+error: generic value parameter 'K' must be a compile-time integer constant
+  --> examples/1137-diagnostics-value-param-type-fn-no-cascade.sx:24:15
+   |
+24 |     b : MakeC(get(), s64) = ---;
+   |               ^^^^^

examples/expected/1137-diagnostics-value-param-type-fn-no-cascade.stdout

@@ -0,0 +1 @@
+

issues/0083-named-const-array-dimension-miscompiled.md

@@ -195,6 +195,23 @@
 > u32 + s8 overflow), `examples/1136-diagnostics-array-dim-nonconst-direct-no-crash.sx`
 > (direct non-const dim halts cleanly, no fabrication / panic); the cascade
 > cleanup also tightened `examples/1502`/`1503` to one diagnostic each.
+>
+> **Final convergence — type-fn binder parity (attempt 10).** One last cell of
+> the count surface still diverged from the struct binder. A FAILED value-param
+> bind on a type-RETURNING FUNCTION (`MakeC :: ($K: Count, $T: Type) -> Type
+> { return [K]T; }`; `a : MakeC(5_000_000_000, s64)`) emitted its correct range
+> diagnostic, but `instantiateTypeFunction` then returned `null`, so
+> `resolveParameterizedWithBindings` fell through to the empty-struct *placeholder*
+> named `MakeC`. The binding `a` got that placeholder type, so a downstream
+> `a.len` cascaded a bogus second error `field 'len' not found on type 'MakeC'`.
+> The struct binder (`instantiateGenericStruct`) already returned `.unresolved`
+> here; the type-fn binder now matches it — the failed value-param bind poisons to
+> `.unresolved` instead of `null`, so the caller propagates the diagnosed poison
+> and the existing `emitFieldError` suppression yields ONE clean diagnostic. Covers
+> every type-fn value-param failure mode (overflow via aliased constraint,
+> non-const arg, unknown type arg). Files: `src/ir/lower.zig` (one line in
+> `instantiateTypeFunction`). Regression:
+> `examples/1137-diagnostics-value-param-type-fn-no-cascade.sx`.
 
 ## Symptom
 A fixed array whose dimension is a module-global integer constant (`N :: 16;

src/ir/lower.zig

@@ -12183,9 +12183,14 @@ pub const Lowering = struct {
                 name_parts.appendSlice(self.alloc, tname) catch {};
             } else {
                 // Value param (e.g., $N: u32) — fold to a compile-time integer
-                // and range-check against its declared type.
+                // and range-check against its declared type. A failed bind has
+                // already diagnosed itself, so poison to `.unresolved` rather
+                // than `null`: `null` makes the caller fall through to the
+                // empty-struct placeholder named after the fn, which then
+                // cascades a bogus `field not found` on any later access. The
+                // struct binder (`instantiateGenericStruct`) poisons the same way.
                 const vp_type: ?[]const u8 = if (tp.constraint.data == .type_expr) tp.constraint.data.type_expr.name else null;
-                const val = self.resolveValueParamArg(args[i], tp.name, vp_type) orelse return null;
+                const val = self.resolveValueParamArg(args[i], tp.name, vp_type) orelse return .unresolved;
                 cvb.put(tp.name, val) catch {};
                 var val_buf: [32]u8 = undefined;
                 const val_str = std.fmt.bufPrint(&val_buf, "{d}", .{val}) catch "0";