fix(diagnostics): make reserved-type-name binding check exhaustive (issue 0076)

The reserved/builtin-type-name binding diagnostic was a hand-walked subset
of binding-bearing AST nodes with a silent `else => {}`, so each review
found another syntactic binding form that bypassed it and hit the original
LLVM verifier abort: destructure names (`s2, x := …`), `impl` method
params/locals, and `if` / `while` / `for` / match-arm / `catch` / `onfail`
captures.

Rewrite `checkBindingNames` (src/ir/semantic_diagnostics.zig) as an
EXHAUSTIVE `switch` over every `Node.Data` tag with NO `else` arm — a future
binding-bearing node type now fails to compile until it is handled here, so
coverage is enforced by the compiler instead of a hand-maintained list. The
check stays in the pre-lowering semantic pass rather than moving to the
`Scope.put` scope-registration choke point: lowering is lazy, so an
uncalled function's bindings never reach `Scope.put`, yet they must still be
rejected at their declaration (e.g. the never-called `takes_u8` in 1119).
No lowering special-case; `lower.zig` unchanged.

Regression tests (fail-before: LLVM abort or silent accept → pass-after:
clean diagnostic, exit 1):
- 1121 control-flow: destructure, if/while bindings, for capture+index,
  match-arm capture
- 1122 impl-block method: reserved param AND reserved local
- 1123 catch + onfail tag bindings
- 1124 destructure name reserved in an imported module
Existing 0125 / 1119 / 0135 / 1120 tests kept; full suite 368 passed.

examples/expected/1121-diagnostics-reserved-name-control-flow.exit

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

examples/expected/1121-diagnostics-reserved-name-control-flow.stderr

@@ -0,0 +1,41 @@
+error: 's2' is a reserved type name and cannot be used as an identifier
+  --> examples/1121-diagnostics-reserved-name-control-flow.sx:18:5
+   |
+18 |     s2, rest := pair();                  // destructure name
+   |     ^^^^^^^^^^^^^^^^^^^
+
+error: 'u8' is a reserved type name and cannot be used as an identifier
+  --> examples/1121-diagnostics-reserved-name-control-flow.sx:19:5
+   |
+19 |     if u8 := maybe() { }                 // if optional binding
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: 's16' is a reserved type name and cannot be used as an identifier
+  --> examples/1121-diagnostics-reserved-name-control-flow.sx:20:5
+   |
+20 |     while s16 := maybe() { break; }      // while optional binding
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: 'bool' is a reserved type name and cannot be used as an identifier
+  --> examples/1121-diagnostics-reserved-name-control-flow.sx:22:5
+   |
+22 |     for xs: (bool) { }                   // for capture name
+   |     ^^^^^^^^^^^^^^^^^^
+
+error: 's32' is a reserved type name and cannot be used as an identifier
+  --> examples/1121-diagnostics-reserved-name-control-flow.sx:23:5
+   |
+23 |     for xs: (v, s32) { }                 // for index name
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: 'string' is a reserved type name and cannot be used as an identifier
+  --> examples/1121-diagnostics-reserved-name-control-flow.sx:25:10
+   |
+25 |     r := if opt == {                     // match-arm capture
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+26 |         case .some: (string) { 0 }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+27 |         case .none: { 0 }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+28 |     };
+   | ^^^^^

examples/expected/1121-diagnostics-reserved-name-control-flow.stdout

@@ -0,0 +1 @@
+

examples/expected/1122-diagnostics-reserved-name-impl-method.exit

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

examples/expected/1122-diagnostics-reserved-name-impl-method.stderr

@@ -0,0 +1,11 @@
+error: 'u8' is a reserved type name and cannot be used as an identifier
+  --> examples/1122-diagnostics-reserved-name-impl-method.sx:19:24
+   |
+19 |     go :: (self: *Box, u8: s64) {
+   |                        ^^
+
+error: 's2' is a reserved type name and cannot be used as an identifier
+  --> examples/1122-diagnostics-reserved-name-impl-method.sx:20:9
+   |
+20 |         s2 := Box.{ total = 1 };
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1122-diagnostics-reserved-name-impl-method.stdout

@@ -0,0 +1 @@
+

examples/expected/1123-diagnostics-reserved-name-catch-onfail.exit

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

examples/expected/1123-diagnostics-reserved-name-catch-onfail.stderr

@@ -0,0 +1,11 @@
+error: 's64' is a reserved type name and cannot be used as an identifier
+  --> examples/1123-diagnostics-reserved-name-catch-onfail.sx:20:5
+   |
+20 |     onfail s64 { }                  // onfail tag binding
+   |     ^^^^^^^^^^^^^^
+
+error: 'u8' is a reserved type name and cannot be used as an identifier
+  --> examples/1123-diagnostics-reserved-name-catch-onfail.sx:21:5
+   |
+21 |     must(n) catch u8 { return; };   // catch tag binding
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1123-diagnostics-reserved-name-catch-onfail.stdout

@@ -0,0 +1 @@
+

examples/expected/1124-diagnostics-imported-reserved-destructure.exit

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

examples/expected/1124-diagnostics-imported-reserved-destructure.stderr

@@ -0,0 +1,5 @@
+error: 's2' is a reserved type name and cannot be used as an identifier
+  --> examples/1124-diagnostics-imported-reserved-destructure/mod.sx:6:5
+   |
+ 6 |     s2, rest := pair();   // destructure name in an IMPORTED module
+   |     ^^^^^^^^^^^^^^^^^^^

examples/expected/1124-diagnostics-imported-reserved-destructure.stdout

@@ -0,0 +1 @@
+