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/0125-types-type-named-var-rejected.sx

@@ -0,0 +1,13 @@
+// A local declared with a reserved/builtin type-name spelling (`s2` is the
+// arbitrary-width `sN` integer type) is rejected at the declaration site.
+// Previously such a name parsed as a `.type_expr`, so address-of sites
+// mis-lowered it (load-by-value to a `ptr` param → LLVM verifier abort, or a
+// silent `*self`-mutation-losing copy). Regression (issue 0076). Expected:
+// error at the declaration; exit 1.
+#import "modules/std.sx";
+
+main :: () -> s32 {
+    s2 := 42;
+    print("s2: {}\n", s2);
+    return 0;
+}

examples/0125-types-type-named-vars.sx

@@ -1,19 +0,0 @@
-#import "modules/std.sx";
-#import "modules/math/math.sx";
-#import "modules/compiler.sx";
-#import "modules/test.sx";
-pkg :: #import "modules/testpkg";
-
-main :: () {
-
-    // ========================================================
-    // 21. TYPE-NAMED VARIABLES (s2, u8, etc.)
-    // ========================================================
-    print("=== 21. Type-Named Vars ===\n");
-    {
-        s2 := 42;
-        print("s2: {}\n", s2);
-        s2 = s2 + 1;
-        print("s2+1: {}\n", s2);
-    }
-}

examples/0135-types-self-streaming-nonreserved.sx

@@ -0,0 +1,30 @@
+// A `*self`-mutating streaming pattern with NON-reserved binding names
+// (`hasher`, `ctx`) compiles and accumulates state correctly through BOTH
+// call styles — explicit address-of `update(@h, ...)` and autoref
+// `h.update(...)` — across multiple mutating calls. Proves the
+// `.identifier`-only address-of paths in lowering are correct as-is, with no
+// type-shaped-name special-case (companion to the issue-0076 rejection of
+// type-named identifiers).
+#import "modules/std.sx";
+
+Hasher :: struct { total: s64 = 0; count: s64 = 0; }
+
+update :: (self: *Hasher, n: s64) {
+    self.total += n;
+    self.count += 1;
+}
+
+main :: () -> s32 {
+    hasher := Hasher.{ total = 0, count = 0 };
+    update(@hasher, 10);   // explicit address-of receiver
+    hasher.update(20);     // autoref receiver
+    update(@hasher, 30);
+    hasher.update(40);
+    print("hasher total={} count={}\n", hasher.total, hasher.count);
+
+    ctx := Hasher.{ total = 100, count = 0 };
+    ctx.update(5);
+    update(@ctx, 7);
+    print("ctx total={} count={}\n", ctx.total, ctx.count);
+    return 0;
+}

examples/0904-optionals-any-to-string-optional.sx

@@ -23,9 +23,9 @@ S :: struct {
 }
 
 main :: () {
-    s1 := S.{ a = 42, b = "hi", c = true };
-    print("{}\n", s1);
-    s2 := S.{ a = null, b = null, c = null };
-    print("{}\n", s2);
+    filled := S.{ a = 42, b = "hi", c = true };
+    print("{}\n", filled);
+    empty := S.{ a = null, b = null, c = null };
+    print("{}\n", empty);
     0;
 }

examples/1119-diagnostics-reserved-type-name-as-identifier.sx

@@ -0,0 +1,16 @@
+// A value binding (parameter or local `var`) spelled as a reserved/builtin
+// type name is rejected at the declaration site, across every declaration
+// form: a parameter name (`u8`), a typed local (`s64`, `bool`), and a `:=`
+// local (`string`). Such a spelling parses as a `.type_expr` rather than an
+// `.identifier`, so the address-of family in lowering mis-lowers it (issue
+// 0076). Expected: one error per offending name; exit 1.
+#import "modules/std.sx";
+
+takes_u8 :: (u8: s32) -> s32 { return u8; }
+
+main :: () -> s32 {
+    s64 : s32 = 3;
+    bool : bool = true;
+    string := "x";
+    return 0;
+}

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";
+   |     ^^^^^^^^^^^^^^