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/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 @@
+