Follow-ups from #803: overlapping Shutdown calls share the shortest budget, the Engine.Shutdown doc sentence, a test margin nit

[FumingPower3925]

Follow-ups from the round-1 review of #803 (celeris#753), none blocking.

1. Overlapping Shutdown calls: the shortest budget ends the shared drain for every caller. Each caller's AfterFunc cancels drainCtx (engine/std/engine.go at 325d83a, about line 246). A caller whose own ctx is still live then gets the internal drainCtx's context.Canceled from http.Server.Shutdown(drainCtx) (the return err at about line 260), since ctx.Err() is nil for it. Before fix(std): a cancel of StartWithContext's context keeps Config.ShutdownTimeout (celeris#753) #803 the winner's budget governed. Unlikely call pattern, misleading error: return nil (or a documented error) to a caller whose own ctx has not expired, or say so in the Shutdown doc. Static reading; not run.
2. engine.Engine.Shutdown's doc at fix(std): a cancel of StartWithContext's context keeps Config.ShutdownTimeout (celeris#753) #803's head says "No engine closes a connection whose handler is still running when ctx expires; the handler runs to completion". That is false on epoll, io_uring and adaptive for an HTTP/2 stream on the shared worker pool (celeris#759). fix(epoll, iouring, std): graceful shutdown waits for the HTTP/2 streams on the shared worker pool, and std for its h2c streams (celeris#759) #808 (which carries fix(std): a cancel of StartWithContext's context keeps Config.ShutdownTimeout (celeris#753) #803) rewrites the doc; if fix(std): a cancel of StartWithContext's context keeps Config.ShutdownTimeout (celeris#753) #803 merges alone, the sentence stays wrong until fix(epoll, iouring, std): graceful shutdown waits for the HTTP/2 streams on the shared worker pool, and std for its h2c streams (celeris#759) #808 does.
3. Nit: el < budget in engine/std/listen_cancel_budget_test.go (about lines 133-141) has a sub-microsecond theoretical margin: shutCtx is created before start is taken. Creating the context after start removes it.