net/traffic: switch rendezvous hashing from SHA256 to FNV-1a (#19821)

In PR tailscale/corp#30448, we originally decided to break ties using
SHA256 for our rendezvous hashing algorithm. Now that we’ve had some
experience with it, we think that FNV-1a is a better choice. It
distributes bits evenly, it’s much faster, and it doesn’t need to be
cryptographically secure. The FNV designers recommend FNV-1a over the
deprecated FNV-1.

This PR makes the switch and updates the related tests, since changing
the algorithm changes which stable pick gets selected. As of 2026-05,
this is the best time to make this change, since there are almost no
clients in the wild with traffic steering enabled.

Updates #17366
Updates tailscale/corp#29964
Updates tailscale/corp#29966
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

ipn/ipnlocal/local_test.go

@@ -5832,8 +5832,8 @@ func TestSuggestExitNodeTrafficSteering(t *testing.T) {
 				},
 			},
 			// Change this, if the hashing function changes.
-			wantID:   "stable3",
-			wantName: "peer3",
+			wantID:   "stable1",
+			wantName: "peer1",
 		},
 		{
 			name: "exit-nodes-without-priority-for-suggestions",
@@ -5973,8 +5973,9 @@ func TestSuggestExitNodeTrafficSteering(t *testing.T) {
 						withLocationPriority(2)), // top
 				},
 			},
-			wantID:   "stable5",
-			wantName: "peer5",
+			// Change this, if the hashing function changes.
+			wantID:   "stable2",
+			wantName: "peer2",
 			wantPri:  2,
 		},
 		{