serial: headless mode tying serial port to stdio for OS debugging

- --headless: no rendering/TUI; serial TX -> stdout, RX <- stdin (nonblocking)
- generalize serial: serial_out_fd/serial_in_fd on the GB; internal-clock
  transfer now shifts in a real RX byte (from stdin) instead of always 0xFF
- paced to native speed by default, --uncapped for turbo; pipes/scripts cleanly
- README: document headless serial console

1 files changed, 2 insertions, 0 deletions

diff --git a/src/gb.h b/src/gb.h
index 7439615..7218648 100644
--- a/src/gb.h
+++ b/src/gb.h
@@ -114,6 +114,8 @@ struct GB {
 
     u64 cycles;        // total T-cycles elapsed
     bool serial_log;   // print serial output to stderr (blargg tests)
+    int  serial_out_fd; // if >=0, transmitted serial bytes written here (stdout)
+    int  serial_in_fd;  // if >=0, received serial bytes read here (stdin), else 0xFF
 };
 
 void gb_init(GB *gb);