fix(diagnostics): reject reserved/builtin type names used as identifiers (issue 0076)

A value binding (local/global `var` or a parameter) spelled as a
reserved/builtin type name parses as a `.type_expr` rather than an
`.identifier` (parser.zig, via `Type.fromName`), so the address-of
family in lower.zig never saw a scoped local and mis-lowered it —
loading the aggregate and passing it by value to a `ptr` parameter
(LLVM verifier abort, or a silent `*self`-mutation-losing copy).

Add a declaration-site diagnostic in semantic_diagnostics.zig
(`UnknownTypeChecker.checkBindingName`): reject any parameter name or
`var` binding name (`:=` / typed-local / global forms) whose spelling
collides with a reserved type name. `isReservedTypeName` defers to the
parser's own classifier (`types.Type.fromName`) so the rejected set
never drifts from the set that would parse as a type — the named
builtins (bool/string/void/f32/f64/usize/isize/Any) and `[su]N` over
sx's 1-64 range. Bare value names (`s`, `self`, `index`) are untouched.
No lowering special-case; the `.identifier`-only address-of paths are
correct once type-shaped names can never be bound. The rejected
attempt-1 `bareVarName` approach was never landed.

Tests:
- 0125-types-type-named-var-rejected: `:=` form (s2) rejected
  (repurposed from the old test that asserted the now-illegal behavior).
- 1119-diagnostics-reserved-type-name-as-identifier: parameter (u8),
  typed-local (s64, bool), `:=` (string) forms rejected.
- 0135-types-self-streaming-nonreserved: positive — `*self` streaming
  with non-reserved names accumulates correctly via both call styles.
- 0904-optionals: renamed incidental locals s1/s2 -> filled/empty.

examples/expected/0125-types-type-named-var-rejected.exit

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

examples/expected/0125-types-type-named-var-rejected.stderr

@@ -0,0 +1,5 @@
+error: 's2' is a reserved type name and cannot be used as an identifier
+  --> /Users/agra/projects/sx/examples/0125-types-type-named-var-rejected.sx:10:5
+   |
+10 |     s2 := 42;
+   |     ^^^^^^^^^

examples/expected/0125-types-type-named-vars.stderr

@@ -1 +0,0 @@
-

examples/expected/0125-types-type-named-vars.stdout

@@ -1,3 +0,0 @@
-=== 21. Type-Named Vars ===
-s2: 42
-s2+1: 43

examples/expected/0135-types-self-streaming-nonreserved.stdout

@@ -0,0 +1,2 @@
+hasher total=100 count=4
+ctx total=112 count=2

examples/expected/1119-diagnostics-reserved-type-name-as-identifier.exit

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

examples/expected/1119-diagnostics-reserved-type-name-as-identifier.stderr

@@ -0,0 +1,23 @@
+error: 'u8' is a reserved type name and cannot be used as an identifier
+  --> /Users/agra/projects/sx/examples/1119-diagnostics-reserved-type-name-as-identifier.sx:9:14
+   |
+ 9 | takes_u8 :: (u8: s32) -> s32 { return u8; }
+   |              ^^
+
+error: 's64' is a reserved type name and cannot be used as an identifier
+  --> /Users/agra/projects/sx/examples/1119-diagnostics-reserved-type-name-as-identifier.sx:12:5
+   |
+12 |     s64 : s32 = 3;
+   |     ^^^^^^^^^^^^^^
+
+error: 'bool' is a reserved type name and cannot be used as an identifier
+  --> /Users/agra/projects/sx/examples/1119-diagnostics-reserved-type-name-as-identifier.sx:13:5
+   |
+13 |     bool : bool = true;
+   |     ^^^^^^^^^^^^^^^^^^^
+
+error: 'string' is a reserved type name and cannot be used as an identifier
+  --> /Users/agra/projects/sx/examples/1119-diagnostics-reserved-type-name-as-identifier.sx:14:5
+   |
+14 |     string := "x";
+   |     ^^^^^^^^^^^^^^