fix(timer): make the wrap-around top-bit test shift unsigned (1U << 31)

[randomizedcoder]

Bug: undefined 1 << 31 in the NEED_ACK wrap-around test

homa_timer_check_rpc() uses a wrap-around-safe comparison to decide when to send a NEED_ACK, testing the top bit of an unsigned tick difference:

if ((rpc->done_timer_ticks + homa->request_ack_ticks
        - 1 - homa->timer_ticks) & 1 << 31) {

1 << 31 shifts a 1 into the sign bit of a signed int — the result (2³¹) isn't representable in int, which is undefined behavior in C.

Because 1 << 31 is a constant expression, the compiler folds it at compile time, so the runtime behavior is correct on gcc/x86 today (and UBSan's runtime shift check never sees it). It remains UB on paper and a portability trap, and -Wshift-overflow=2 flags it.

Fix

-					- 1 - homa->timer_ticks) & 1 << 31) {
+					- 1 - homa->timer_ticks) & 1U << 31) {

1U << 31 is well-defined unsigned. The operands are already unsigned (done_timer_ticks/timer_ticks are u32, so the whole expression is unsigned int), so the masked value — the top bit of the wrap difference — is identical.

Verification (compiler gate + regression)

Before:  ../homa_timer.c:37: warning: result of '1 << 31' requires 33 bits to
         represent, but 'int' only has 32 bits [-Wshift-overflow=]
After:   (clean — no homa_timer.c warning)

-Wshift-overflow=2. The timer unit suite stays green (10/10); homa_timer_check_rpc__request_ack still walks its tick sequence to "xmit NEED_ACK", confirming the behavior is unchanged.

Note on the gate: the initial plan expected UBSan to catch this, but a constant-folded shift produces no runtime shift for UBSan to instrument — so the authoritative gate here is the compiler's -Wshift-overflow=2, backed by the unchanged behavioral test.