fix(types): extend unknown-type check into function bodies (issue 0064)

The signature/field check missed body-level type positions: a local
annotation naming a non-existent type flowed through the empty-struct stub
untouched, so `v: Coordnate = 5` silently compiled and ran (the value
dropped) — an invalid program accepted with no diagnostic.

`checkUnknownTypeNames` now also walks each main-file function body
(`checkBodyTypes`): local var/const type annotations — including inside
if / loop / match / push / defer / onfail blocks and decl-value blocks — are
validated with the enclosing function's generic params in scope, and
body-local `T :: struct/enum/union` declarations are collected first
(`collectBodyDeclNames`) so legitimate locals aren't false-flagged. Nested
function/closure bodies are their own scope and are not descended (safe
under-coverage); explicit `cast(T)` already surfaces its own `unresolved`
diagnostic and is left to it.

Regression: examples/1113 (local annotation of a non-existent type, exit 1).

examples/1113-diagnostics-unknown-type-local-var-rejected.sx

@@ -0,0 +1,10 @@
+// An identifier used in a LOCAL variable's type annotation that names no
+// declared type is rejected, exactly like a signature or struct-field use.
+// Previously a body-level annotation bypassed the check: the type resolver's
+// empty-struct stub silently gave the local a 0-field type, so `v: Coordnate
+// = 5` compiled and ran (the `5` dropped) with no diagnostic. Regression
+// (issue 0064, body-level positions). Expected: error at the annotation; exit 1.
+main :: () -> s32 {
+    v: Coordnate = 5;
+    return 0;
+}

examples/expected/1113-diagnostics-unknown-type-local-var-rejected.exit

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

examples/expected/1113-diagnostics-unknown-type-local-var-rejected.stderr

@@ -0,0 +1,5 @@
+error: unknown type 'Coordnate'
+  --> /Users/agra/projects/sx/examples/1113-diagnostics-unknown-type-local-var-rejected.sx:8:8
+   |
+ 8 |     v: Coordnate = 5;
+   |        ^^^^^^^^^

examples/expected/1113-diagnostics-unknown-type-local-var-rejected.stdout

@@ -0,0 +1 @@
+