Make select wakeups record an explicit winner and wake reason

[drpaneas]

Summary

Blocked select currently reconstructs the winning case after wakeup by scanning sudog.success. That is too weak for close-wakeup semantics and makes selection outcome ambiguous.

Why

• Channel close wakes blocked receivers and senders with success = false.
• select then has no explicit case winner for wakeups caused by close.
• This risks subtle semantic bugs in one of the most correctness-sensitive parts of the runtime.

Evidence

• runtime/select.c: selectgo() finds the selected case by looking for sg->success == true after wakeup.
• runtime/chan.c: closechan() wakes both recvq and sendq waiters with sg->success = false.
• runtime/select.c: the post-wakeup closed-channel handling still assumes a selected case index exists.

Direction

• Store selected case index explicitly in the blocked goroutine state.
• Add an explicit wake reason such as send, recv, closed, or cancelled.
• Keep success for data-path bookkeeping only, not for deciding which case won.