fix(types): check nested closure/function bodies and cast targets (issue 0064)

Closes the two residual silent holes in the unknown-type diagnostic:

- Nested closure / function bodies. The body walk stopped at closure and
  nested-fn boundaries, so a typo'd type in a closure's local annotation
  silently became a 0-field struct. `walkBodyTypes` now descends control
  flow and expressions to re-enter each closure / nested fn via `checkScope`,
  which accumulates that scope's generic + value-`Type` params onto the
  parent's — so an inner closure still sees the outer function's `$T` (no
  false positive) while a genuine unknown is flagged at any nesting depth.
  `harvestScopeDecls` collects type-decl names across the whole body
  (including nested scopes) up front so locals are never false-flagged.

- Cast targets. `cast(T)` where `T` is a value-`Type` param (no `$`) cast to
  a fabricated empty struct silently; it now gets the tailored `$T` hint. An
  unknown *literal* cast target already errors via value resolution, so it's
  left to that path — no double diagnostic.

Suite: 350 passed, 0 failed. Regressions: examples/1114 (nested-closure
annotation), 1115 (cast value param).

examples/1114-diagnostics-unknown-type-nested-closure-rejected.sx

@@ -0,0 +1,12 @@
+// An unknown type in a NESTED closure body's local annotation is rejected,
+// with the enclosing function's generic params still in scope. Previously the
+// body walk stopped at closure / nested-function boundaries, so a typo'd type
+// inside a closure slipped through and silently became a 0-field struct.
+// Regression (issue 0064, nested scopes). Expected: error at the annotation; exit 1.
+main :: () -> s32 {
+    f := closure(() -> s32 {
+        bad: Coordnate = ---;
+        return 0;
+    });
+    return f();
+}

examples/1115-diagnostics-cast-value-param-rejected.sx

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+// `cast(T)` where `T` is a value parameter declared `: Type` (without the `$`
+// generic sigil) is rejected with the generic-param hint. Previously it
+// silently cast to a fabricated empty struct (an unknown *literal* cast target
+// already errored via value resolution, but the value-param case was silent).
+// Regression (issue 0064, cast position). Expected: tailored error; exit 1.
+conv :: (T: Type, x: s32) -> s32 {
+    return cast(T) x;
+}
+
+main :: () -> s32 {
+    return conv(s32, 5);
+}

examples/expected/1114-diagnostics-unknown-type-nested-closure-rejected.exit

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+1

examples/expected/1114-diagnostics-unknown-type-nested-closure-rejected.stderr

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+error: unknown type 'Coordnate'
+  --> /Users/agra/projects/sx/examples/1114-diagnostics-unknown-type-nested-closure-rejected.sx:8:14
+   |
+ 8 |         bad: Coordnate = ---;
+   |              ^^^^^^^^^

examples/expected/1114-diagnostics-unknown-type-nested-closure-rejected.stdout

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+

examples/expected/1115-diagnostics-cast-value-param-rejected.exit

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+1

examples/expected/1115-diagnostics-cast-value-param-rejected.stderr

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+error: 'T' is a value parameter, not a type; introduce a generic type parameter with `$T: Type`
+  --> /Users/agra/projects/sx/examples/1115-diagnostics-cast-value-param-rejected.sx:7:17
+   |
+ 7 |     return cast(T) x;
+   |                 ^

examples/expected/1115-diagnostics-cast-value-param-rejected.stdout

@@ -0,0 +1 @@
+