fix: bindingless if/while/and/or over optional reads has_value (issue 0164)

lowerIfExpr emitted optional_has_value only for the binding form; a bare
'if opt' passed the raw {T,i1} aggregate to condBr, where emitCondBr's
catch-all struct arm silently folded it to 'i1 true' (structs always
truthy) — a silent miscompile that took the present-branch for null
optionals. while / and / or shared the same defect.

Reduce bindingless optional conditions to optional_has_value in
lowerIfExpr/lowerWhile and via a new lowerBoolCondition helper for and/or
operands. Replace the silent-true emitCondBr arm with a lowering-time
diagnostic (checkConditionType/isValidConditionType) rejecting conditions
whose type isn't bool/integer/pointer/optional; the backend @panic is now
an unreachable tripwire.

Regressions: examples/optionals/0908..0910 + diagnostics/1194 (negative).
Verified by 3+3 adversarial reviews.

Filed adjacent bugs found during review: 0168 (array-of-optionals element
load), 0169 (optional->bool coercion), 0170 (closure-optional layout).

examples/optionals/0909-optionals-while-no-binding.sx

@@ -0,0 +1,33 @@
+// Bindingless `while <optional>` must test the optional's has_value flag,
+// exactly like `if <optional>` — not fold the `{T,i1}` aggregate truthy.
+//
+// Regression (issue 0164): a bare optional loop condition emitted `br i1 true`,
+// so the loop never observed `null` and either spun forever (present→drained)
+// or ran when it should have been skipped (already-null).
+#import "modules/std.sx";
+
+main :: () {
+    // Present `?i64` drained to null: the body runs each step until the
+    // optional becomes null, then the loop stops.
+    countdown : ?i64 = 3;
+    runs := 0;
+    while countdown {
+        runs = runs + 1;
+        v := countdown!;          // unwrap the present value
+        if v <= 1 {
+            countdown = null;     // drain → loop must STOP next header eval
+        } else {
+            countdown = v - 1;
+        }
+    }
+    print("drain runs={}\n", runs);     // 3
+
+    // Already-null `?i64`: the body must NOT run at all.
+    empty : ?i64 = null;
+    skipped := 0;
+    while empty {
+        skipped = skipped + 1;
+        empty = null;
+    }
+    print("skip runs={}\n", skipped);   // 0
+}