fix(diag): generic VALUE param ($N: u32) used as a field/annotation type → diagnostic (no .unresolved LLVM panic) [stdlib E3 attempt-3]

The generic-struct field checker (attempt-2) accepted ALL struct type
params as valid type-name leaves, including VALUE params. The parser
marks any reference to a struct's own param `is_generic` (so `x: T`
resolves without `$`), and it marks a value param `$N: u32` the same
way — so `Bad :: struct($N: u32) { x: N; }` instantiated `Bad(3)` slipped
past the unknown-type walk, resolved the field's type leaf to the
`.unresolved` sentinel, and panicked at LLVM emission instead of
diagnosing.

Distinguish TYPE params (`$T: Type`, `$T: SomeProtocol`, the `..$Ts`
pack) from VALUE params (`$N: u32`) using the binder's own classification
rule (lower.zig). A value param named in a type position now gets the
tailored "'N' is a value parameter, not a type" hint, exit 1, before
codegen. Two dispatch paths covered: the `is_generic` struct-field path
(reportIfValueParamInTypePosition) and the non-generic annotation path
(reportIfUnknownType in-scope filter). A value param in a VALUE position
(array dim `[N]u8`, `Vector` lane) still resolves.

Regression: 0172-types-value-param-as-field-type (panic-before / clean
diagnostic-after). 0171 and 0759 stay green; 499 markers, prior
byte-identical.

examples/0172-types-value-param-as-field-type.sx

@@ -0,0 +1,29 @@
+// A generic struct's VALUE param (`$N: u32`) is a compile-time integer, not a
+// type. Naming it in a TYPE position — here a field type `x: N` — must emit a
+// clean diagnostic, NOT silently compile.
+//
+// The parser marks any reference to a struct's own type param `is_generic`
+// (so `x: T` for a real `$T: Type` resolves without a `$`). That marking is
+// the same for a value param, so the unknown-type walk used to skip `x: N`
+// entirely; the field's type leaf then resolved to the `.unresolved` sentinel
+// and PANICKED at LLVM emission ("unresolved type reached LLVM emission").
+//
+// The checker now distinguishes TYPE params (`$T: Type`, `$T: SomeProtocol`,
+// the `..$Ts` pack) from VALUE params (`$N: u32`) using the binder's own rule:
+// a value param named in a type position gets the tailored hint. A value param
+// in a VALUE position (a `[N]u8` array dimension, a `Vector` lane count) still
+// works (see 0147 / 0201).
+//
+// Expected: `error: 'N' is a value parameter, not a type`; exit 1.
+// Regression (stdlib E3).
+#import "modules/std.sx";
+
+Bad :: struct($N: u32) {
+    x: N;
+}
+
+main :: () -> s32 {
+    b : Bad(3) = .{ x = 1 };
+    print("{}\n", b.x);
+    return 0;
+}

examples/expected/0172-types-value-param-as-field-type.exit

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

examples/expected/0172-types-value-param-as-field-type.stderr

@@ -0,0 +1,5 @@
+error: 'N' is a value parameter, not a type; introduce a generic type parameter with `$N: Type`
+  --> examples/0172-types-value-param-as-field-type.sx:22:8
+   |
+22 |     x: N;
+   |        ^

examples/expected/0172-types-value-param-as-field-type.stdout

@@ -0,0 +1 @@
+