net: one socket per client - four-tuple demux, spawning accept, bounded close

A server could only ever hold ONE connection, which makes an ircd pointless:
listen() turned the listener into the connection, and net_find_tcp demuxed on
local port alone, so two clients on 6667 were indistinguishable.

- net_find_tcp now matches an established socket on the full four-tuple
  (local port + peer IP + peer port) and only falls back to a LISTENing
  socket when none matches.
- A SYN at a listener no longer consumes it: tcp_spawn_conn clones a new
  socket (local port + owning pid, so exit/kill still reclaims it) and the
  listener keeps listening. accept() returns that socket; closing it leaves
  the listener alone. A full table drops the SYN, and the client's retransmit
  is taken once a slot frees.
- tcp_peer_is_current preserves BC: net_find_tcp scans with the socket index
  there, and the compare needs BC for the rx buffer base. Without this the
  scan died at the first mismatching socket, so a second client's handshake
  ACK never reached its socket and it hung in SYNRCVD - tcpdump showed our
  SYN+ACK going out and the client's NICK retransmitted five times into
  silence.
- listen() only conflicts with another LISTENER on the port; connections
  share it by design now.
- MAX_SOCKS 4 -> 6 (listener + 3 clients + 2 spare; 537 bytes of WRAM0 still
  free), and SK_ACC marks a spawned connection as not yet accepted.

tcp_close's settle is also time-boxed to ~250 ms of real time. It counted
PUMPS (8 x 8000), which is ~30s of wall clock when the peer never answers -
invisible with well-behaved peers, but a daemon refusing a client that stayed
connected froze itself, and every other client with it, for half a minute.

httpd keeps its listener open across requests now, so the next client's SYN
is accepted immediately instead of being dropped between close and re-listen:
three parallel clients go 0.1/1.1/2.1s -> 0.4/1.0/0.1s.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index e0961bc..0ed9f85 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -282,7 +282,7 @@ ENDR
 ; (TimerISR can carry between the two byte reads, which would otherwise read
 ; $01FF out of a counter going $00FF -> $0100).
 ; -----------------------------------------------------------------------------
-ticks16:
+ticks16::
     ld a, [wTicks+1]
     ld h, a
     ld a, [wTicks]