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/diagnostics/1194-diagnostics-condition-non-bool-type.sx

@@ -0,0 +1,16 @@
+// A branch condition (`if` / `while` / `and` / `or`) must reduce to an i1:
+// its type must be a bool, integer, pointer, or optional. A struct (or float,
+// etc.) has no truthiness — it used to be silently folded truthy at lowering
+// then `@panic` in the LLVM backend (issue 0164). It must instead be a clean,
+// located compile-time TYPE error.
+//
+// Negative test: locks the new diagnostic. `if <struct>` is rejected.
+#import "modules/std.sx";
+
+S :: struct { x: i64; }
+
+main :: () -> i64 {
+    s : S = .{ x = 1 };
+    if s { return 1; }   // ERROR: condition must be a bool, integer, pointer, or optional
+    return 0;
+}

examples/diagnostics/expected/1194-diagnostics-condition-non-bool-type.exit

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

examples/diagnostics/expected/1194-diagnostics-condition-non-bool-type.stderr

@@ -0,0 +1,5 @@
+error: condition must be a bool, integer, pointer, or optional, but has type 'S'
+  --> examples/diagnostics/1194-diagnostics-condition-non-bool-type.sx:14:8
+   |
+14 |     if s { return 1; }   // ERROR: condition must be a bool, integer, pointer, or optional
+   |        ^

examples/diagnostics/expected/1194-diagnostics-condition-non-bool-type.stdout

@@ -0,0 +1 @@
+