issue 0152 RESOLVED: byte-promote sub-byte (Atomic(bool)) atomic load/store

LLVM rejects a sub-byte atomic memory access (must be byte-sized), so
Atomic(bool) — bool lowers to i1 — failed verification on load/store. The
atomic emitters in src/backend/llvm/ops.zig now perform a sub-byte access in
its byte storage type (i8) and trunc/zext the value at the boundary (new
atomicByteType helper: i8 for .bool, null otherwise). rmw/cmpxchg are left
as-is on purpose — a bool rmw/CAS is rejected at the sx level (integer-only),
so a sub-byte element never reaches those emitters.

Regression test examples/1705-atomics-bool-byte-promoted.sx. Suite green 729/0.
Unblocks Future.canceled: Atomic(bool) in the B1.2 async layer.

examples/1705-atomics-bool-byte-promoted.sx

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+// Atomic(bool) — a sub-byte (i1) element atomically loaded/stored. LLVM
+// rejects a sub-byte atomic ("atomic memory access' size must be byte-
+// sized"), so codegen performs the access in the byte storage type (i8)
+// and trunc/zext's the value at the boundary. (rmw/cmpxchg on a bool is
+// rejected at the sx level — integer-only — so only load/store apply.)
+// Regression (issue 0152): Atomic(bool) emitted an i1 atomic that failed
+// LLVM verification; Future.canceled: Atomic(bool) in the async layer hit it.
+#import "modules/std.sx";
+#import "modules/std/atomic.sx";
+
+main :: () {
+    a := Atomic(bool).init(false);
+    print("init: {}\n", a.load(.acquire));        // false
+
+    a.store(true, .release);
+    print("after store: {}\n", a.load(.acquire)); // true
+
+    a.store(false, .seq_cst);
+    print("after reset: {}\n", a.load(.seq_cst)); // false
+}

examples/expected/1705-atomics-bool-byte-promoted.exit

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+0

examples/expected/1705-atomics-bool-byte-promoted.stderr

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+

examples/expected/1705-atomics-bool-byte-promoted.stdout

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+init: false
+after store: true
+after reset: false