net: fix truncated TCP field offsets corrupting multi-segment recv

SK_STATE/SK_SND/SK_RCV sat at struct offsets 305/306/310, past the
288-byte SK_RXBUF. But the field accessors add the offset as an 8-bit
immediate (add SK_x; ld l,a), so rgbasm truncated 305->49, 306->50,
310->54 (the -Wtruncation warnings we'd been ignoring) - putting STATE
and the sequence numbers *inside* RXBUF at offsets 49/50/54.

Any TCP segment with >=33 payload bytes therefore overwrote the
connection's own STATE and rcv_nxt/snd_nxt with message text. The first
segment of a stream landed, its data clobbered STATE to a garbage value,
and every subsequent segment was dropped because tcp_in no longer saw
ESTABLISHED - so multi-segment TCP receives (an IRC MOTD, any HTTP body
past one segment) silently stalled. wget appeared to 'work' only because
a single-segment reply plus a never-honored FIN still printed once.

Fix: reorder the struct so STATE/SND/RCV precede the big RXBUF, keeping
every field offset < 256 (SK_SIZE unchanged at 314, all accessors are
symbolic). Zero -Wtruncation warnings remain. Verified an 11-line IRC
registration burst now arrives intact.

1 files changed, 10 insertions, 4 deletions

diff --git a/src/socket.asm b/src/socket.asm
index 20ed982..0720b0a 100644
--- a/src/socket.asm
+++ b/src/socket.asm
@@ -21,10 +21,16 @@ DEF SK_HASRX   EQU 9     ; 1  rxbuf holds a datagram
 DEF SK_RXLEN   EQU 10    ; 1  payload length
 DEF SK_SRCIP   EQU 11    ; 4  sender IP
 DEF SK_SRCPORT EQU 15    ; 2  sender port
-DEF SK_RXBUF   EQU 17    ; 288 payload (holds a DHCP reply, or a max segment)
-DEF SK_STATE   EQU 305   ; 1  TCP state (see TCP_* below)
-DEF SK_SND     EQU 306   ; 4  snd_nxt (our next sequence number)
-DEF SK_RCV     EQU 310   ; 4  rcv_nxt (next sequence we expect from peer)
+; TCP bookkeeping MUST stay below offset 256: the field accessors do
+; `add SK_x` as an 8-bit immediate, so any constant >=256 is silently
+; truncated (e.g. 305 -> 49) and lands *inside* RXBUF, where an incoming
+; segment's payload then overwrites the connection's STATE/seq numbers -
+; corrupting every multi-segment TCP receive. Keep STATE/SND/RCV ahead of
+; the big RXBUF; RXBUF is only ever based via a 16-bit-safe add.
+DEF SK_STATE   EQU 17    ; 1  TCP state (see TCP_* below)
+DEF SK_SND     EQU 18    ; 4  snd_nxt (our next sequence number)
+DEF SK_RCV     EQU 22    ; 4  rcv_nxt (next sequence we expect from peer)
+DEF SK_RXBUF   EQU 26    ; 288 payload (holds a DHCP reply, or a max segment)
 DEF SK_SIZE    EQU 314
 DEF TCP_MSS    EQU 200