net: keep network packets out of the console (fixes shell garbage on netboot)

The link port is both the console-input fallback and the network. At the shell
prompt the host's multicast (mDNS/LLMNR/IGMP) was fed to the serial and KGetc
read those packet bytes as console input -> garbage in the shell, OSK unusable.

Two-part fix:
- KGetc now skips SLIP frames (0xC0-delimited) on the serial console path, so
  inbound packets never surface as console input. Plain injected bytes (the
  headless tunbridge command path) still pass through. New wConInSkip flag.
- netboot/tunbridge only forward IP packets destined to 10.0.0.2 (drop the
  multicast noise at the bridge).

Verified: GB sits at a clean "/#" prompt while all packets (noise included) are
forwarded; being-pinged (3/3) and wget still work. On the emulator, the OSK is
SELECT=space, START=enter, A=z, B=x, d-pad=WASD/arrows.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index 5b40ef0..04e9661 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -137,6 +137,29 @@ KGetc::
     bit 7, a
     jr nz, .yield
     ld a, [rSB]                 ; A = serial byte
+    ; The link port is also the network. Skip SLIP frames (0xC0-delimited) so
+    ; inbound packets never surface as console input; plain bytes (an injected
+    ; command in headless use) still pass through.
+    ld b, a
+    ld a, [wConInSkip]
+    or a
+    jr nz, .inframe
+    ld a, b
+    cp SLIP_END
+    jr z, .frameopen
+    ld a, b                     ; ordinary byte -> console input
+    jr .done
+.frameopen
+    ld a, 1
+    ld [wConInSkip], a
+    jr .poll
+.inframe
+    ld a, b
+    cp SLIP_END
+    jr nz, .poll                ; still inside a frame -> discard
+    xor a
+    ld [wConInSkip], a          ; frame end -> resume
+    jr .poll
 .done
     and a                       ; CF = 0 (not EOF)
     ret