ipn/ipnlocal: use the live peer map, not the netmap's stale Peers slice

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The nodeBackend's netMap.Peers slice is frozen at the last full netmap
install; the live per-peer state lives in the nodeBackend.peers map,
updated by delta mutations. Three spots still read the stale slice or
paid to materialize a fresh one:

AppendMatchingPeers iterated netMap.Peers and re-looked-up each ID in
the peers map (with a lock round-trip per peer), so peers added by a
delta since the last full netmap were invisible to it. That affected
its callers: taildrop's file-target list, exit node suggestions, and
conn25's connector discovery. It now snapshots the peers map directly
(sorted by node ID, matching the old netmap ordering).

DebugPeerDiscoKeys read netMap.Peers and so returned stale disco keys
after deltas. It now reads the peers map via the new
nodeBackend.peerDiscoKeys.

pingPeerAPI called NetMapWithPeers, building and sorting the full
O(n) peer slice, just to linearly scan it for one IP. It now uses the
nodeBackend's existing by-address index and the new O(1) PeerByID
accessor, and passes the peers-free netmap to peerAPIBase, which only
reads the self node's addresses.

Updates #12542

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I2e57527d64733b4eb17006f896faaa907b1d128c

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