fix(0129): logical not is truthiness-aware, not a bit flip

The unary .not arm emitted bool_not (LLVM bitwise Not) for every
operand. Correct on i1; on an error binding — an error-set value, u32
tag at the LLVM level — a bitwise not of a nonzero tag stays nonzero,
so 'if !e' held even on a SET error and its branch read the
uninitialized success value (real segfault in the distribution repo's
sqlite tests). Plain integers had the same hole ('!7' was '~7').

Now: bool keeps bool_not; integers and error-set operands lower as the
truthiness complement (cmp_eq against a typed zero); anything else is
diagnosed instead of silently bit-flipped.

Regression: examples/1057 (set error: !e must not hold; success: !e
holds with a real value; integer truthiness) + examples/1171 (!"text"
diagnosed); both FAIL pre-fix. zig build test 426/426;
tests/run_examples.sh 600/600.

examples/1057-errors-negated-error-binding.sx

@@ -0,0 +1,36 @@
+// `!` on an error binding is the truthiness complement of `if e` (issue
+// 0129). Pre-fix, `!` lowered as a bitwise not, so a nonzero error tag
+// stayed nonzero and `if !e` held even on a SET error — with the success
+// value read as garbage. Integer operands get the same `!x ≡ x == 0`
+// semantics.
+#import "modules/std.sx";
+
+E :: error { Boom }
+
+f :: (fail: bool) -> (i64, !E) {
+    if fail { raise error.Boom; }
+    return 42;
+}
+
+main :: () -> i32 {
+    // set error: `if e` holds, `if !e` must NOT
+    v, e := f(true);
+    took_e := false;
+    if e { took_e = true; }
+    if !e { print("BUG: !e held on a set error (v={})\n", v); return 1; }
+    if !took_e { print("BUG: if e did not hold on a set error\n"); return 2; }
+
+    // success: `if !e` holds and the value is real
+    v2, e2 := f(false);
+    if e2 { print("BUG: e2 set on success\n"); return 3; }
+    if !e2 { print("ok: !e2 on success, v2={}\n", v2); }
+
+    // integers: `!n` is `n == 0`, not a bit flip
+    n := 7;
+    if !n { print("BUG: !7 held\n"); return 4; }
+    z := 0;
+    if !z { print("ok: !0 holds\n"); }
+
+    print("done\n");
+    return 0;
+}

examples/1171-diagnostics-logical-not-bad-operand.sx

@@ -0,0 +1,10 @@
+// `!` on an operand that has no truthiness (neither bool, integer, nor
+// an error binding) is diagnosed instead of silently bit-flipped
+// (issue 0129's diagnostic half).
+#import "modules/std.sx";
+
+main :: () -> i32 {
+    s := "text";
+    if !s { print("unreachable\n"); }
+    return 0;
+}

examples/expected/1057-errors-negated-error-binding.exit

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

examples/expected/1057-errors-negated-error-binding.stderr

@@ -0,0 +1 @@
+

examples/expected/1057-errors-negated-error-binding.stdout

@@ -0,0 +1,3 @@
+ok: !e2 on success, v2=42
+ok: !0 holds
+done

examples/expected/1171-diagnostics-logical-not-bad-operand.exit

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

examples/expected/1171-diagnostics-logical-not-bad-operand.stderr

@@ -0,0 +1,5 @@
+error: '!' needs a bool, integer, or error operand; got 'string'
+  --> examples/1171-diagnostics-logical-not-bad-operand.sx:8:8
+   |
+ 8 |     if !s { print("unreachable\n"); }
+   |        ^^

examples/expected/1171-diagnostics-logical-not-bad-operand.stdout

@@ -0,0 +1 @@
+