fix(types): reject unknown type names instead of silent empty struct (issue 0064)

An identifier used in a type position that resolved to nothing fell through
to `type_bridge.resolveTypeName`'s empty-struct-stub fallback, silently
interning a 0-field struct named after the identifier. A value parameter
mistakenly used as a type (`(T: Type, ...) -> T`, missing the `$`) or a
typo'd type name therefore compiled and ran, rendering as `T{}`.

New post-scan diagnostic pass `checkUnknownTypeNames` (lower.zig Pass 1f)
walks every main-file function signature and non-generic struct field type
and rejects any leaf name that is not a primitive, an in-scope generic param
(`$T` / `type_params`), a declared type, or a real (non-stub) registered
type. The load-bearing empty-struct stub is left intact — forward references
and foreign-class opaque types still depend on it during the scan — and the
pass runs before body lowering, so `hasErrors()` halts the build before any
stub reaches codegen.

A value param used as a type gets a tailored hint to write `$T: Type`; a
genuine unknown gets "unknown type 'X'". Imported concrete types are
recognized via the type table, and inline compound spellings (`[:0]u8`),
arbitrary-width ints (`u1`/`u2`), and `$`-introduced generics (`-> $R`) are
exempted to avoid false positives.

Regressions: examples/1111 (tailored hint) + 1112 (typo'd field type).

examples/1111-diagnostics-nondollar-type-param-rejected.sx

@@ -0,0 +1,10 @@
+// A value parameter declared `T: Type` (WITHOUT the `$` generic sigil) used in
+// a type position is rejected with a hint to write `$T: Type`. Previously the
+// type resolver silently fabricated a 0-field struct named `T`, so the call
+// compiled and rendered as `T{}` at runtime with no diagnostic.
+// Regression (issue 0064). Expected: one error per `T` use site; exit 1.
+idwrap :: (T: Type, f: Closure() -> T) -> T { return f(); }
+
+main :: () -> s32 {
+    return idwrap(s32, closure(() -> s32 { return 7; }));
+}

examples/1112-diagnostics-unknown-type-name-rejected.sx

@@ -0,0 +1,13 @@
+// An identifier used in a type position that names no declared type, builtin,
+// or in-scope generic parameter is rejected. Previously the type resolver's
+// empty-struct-stub fallback silently interned a 0-field struct under the typo,
+// so the program compiled and ran. Regression (issue 0064, broader fix).
+// Expected: a clean "unknown type" error at the field; exit 1.
+Point :: struct {
+    x: s32;
+    y: Coordnate;   // typo for a non-existent type
+}
+
+main :: () -> s32 {
+    return 0;
+}

examples/expected/1111-diagnostics-nondollar-type-param-rejected.exit

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

examples/expected/1111-diagnostics-nondollar-type-param-rejected.stderr

@@ -0,0 +1,11 @@
+error: 'T' is a value parameter, not a type; introduce a generic type parameter with `$T: Type`
+  --> /Users/agra/projects/sx/examples/1111-diagnostics-nondollar-type-param-rejected.sx:6:37
+   |
+ 6 | idwrap :: (T: Type, f: Closure() -> T) -> T { return f(); }
+   |                                     ^
+
+error: 'T' is a value parameter, not a type; introduce a generic type parameter with `$T: Type`
+  --> /Users/agra/projects/sx/examples/1111-diagnostics-nondollar-type-param-rejected.sx:6:43
+   |
+ 6 | idwrap :: (T: Type, f: Closure() -> T) -> T { return f(); }
+   |                                           ^

examples/expected/1111-diagnostics-nondollar-type-param-rejected.stdout

@@ -0,0 +1 @@
+

examples/expected/1112-diagnostics-unknown-type-name-rejected.exit

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

examples/expected/1112-diagnostics-unknown-type-name-rejected.stderr

@@ -0,0 +1,5 @@
+error: unknown type 'Coordnate'
+  --> /Users/agra/projects/sx/examples/1112-diagnostics-unknown-type-name-rejected.sx:8:8
+   |
+ 8 |     y: Coordnate;   // typo for a non-existent type
+   |        ^^^^^^^^^

examples/expected/1112-diagnostics-unknown-type-name-rejected.stdout

@@ -0,0 +1 @@
+