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/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

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+