plugin/shed: add UDP overload protection plugin (#8312)

* plugin/shed: add UDP overload protection plugin

UDP responses written back through one listener socket serialize on the
Go runtime's internal fdMutex, which allows at most 2^20-1 concurrent
operations per file descriptor and panics the process when exceeded.
CoreDNS serves UDP with one goroutine per query, all writing through the
shared packet connection, so a sustained overload parks every excess
in-flight query in that wait queue until the process dies with
"too many concurrent operations on a single file or socket". Observed
in production: ~2.8M goroutines and 60GiB RSS before the panic.

The shed plugin makes the panic structurally unreachable. It installs,
via Config.UDPDecorateWriterFunc, a per-socket bounded evict-oldest
stack drained newest-first by a single writer goroutine, so the fd
never sees more than one writer and residual capacity under overload
always goes to the freshest response. While a socket's stack is full,
arriving queries are dropped before any plugin runs. Drops are silent
(the client's resolver retries elsewhere) and counted in
coredns_shed_dropped_total{server, reason}.

plugin/shed/fdmutex_test.go demonstrates the failure and the fix with
one shared flood harness. Two subprocess tests reproduce the exact
runtime panic without the plugin's write discipline - one deterministic
(a held write plus >2^20 queued writers), one with nothing held or
mocked; both exercise the Go runtime rather than the plugin, so they
are gated behind SHED_FLOOD_TEST=1. The counterfactual - the same load
through the plugin's stack, completing with every response accounted
for as written or dropped - runs in every test invocation, including
-race, at 50k responders, and at the full 1.5M with SHED_FLOOD_TEST=1:

    SHED_FLOOD_TEST=1 go test ./plugin/shed/

Signed-off-by: Ryan Brewster <rpb@anthropic.com>

* test: add shed e2e test

Query a shed-enabled server over UDP (the plugin's deferred
single-writer path) and TCP (which shed passes through), and check
that coredns_shed_dropped_total is exported with its reason label.

No-Verification-Needed: test-only change
Signed-off-by: Ryan Brewster <rpb@anthropic.com>

---------

Signed-off-by: Ryan Brewster <rpb@anthropic.com>

core/dnsserver/zdirectives.go

@@ -31,6 +31,7 @@ var Directives = []string{
 	"ready",
 	"health",
 	"pprof",
+	"shed",
 	"prometheus",
 	"errors",
 	"log",