Add "PC SoG/CS coast" compatibility option

tvp_source_setup() disables the TVP7002 internal H-PLL coast for every
PC-type preset. That is right for RGBHV, but with sync-on-green or
composite sync the serration pulses during vsync disturb the H-PLL every
frame; on a NeXTstation Color (1120x832, SoG) this caused continuous
horizontal jitter, jitter in the top third of the picture and a periodic
HDMI re-sync. With the option on, coast is re-enabled after setup when the
sync source is SOG or CSYNC; RGBHV is untouched. Default Off, so existing
behaviour is unchanged. Stored as profile item id 68.

On the NeXT, this option together with the existing settings Analog sync
LPF = 10 MHz, ADC PLL BW = Low and H-PLL Pre/Post-Coast = 4/4 ran 19
minutes without a sync loss, mode re-program or IT6613 video-stable
change. SPARCstation 20 framebuffers (ZX, TurboGX) over composite sync are
stable without it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WiXHy7Jhh5yTdsBaenucgw

software/sys_controller/ic_drivers/tvp7002/tvp7002.c

@@ -180,6 +180,13 @@ inline void tvp_set_hpllcoast(alt_u8 pre, alt_u8 post)
     tvp_writereg(TVP_HPLLPOSTCOAST, post);
 }
 
+// Enable/disable the internal H-PLL coast signal (MISCCTRL4 bit 2, "Coast Dis")
+void tvp_set_coast_enable(alt_u8 enable)
+{
+    alt_u8 val = tvp_readreg(TVP_MISCCTRL4) & 0xFB;
+    tvp_writereg(TVP_MISCCTRL4, val | (enable ? 0x00 : 0x04));
+}
+
 inline void tvp_set_linelen_tol(alt_u8 val) {
     tvp_writereg(TVP_LINELENTOL, val);
 }

software/sys_controller/ic_drivers/tvp7002/tvp7002.h

@@ -90,6 +90,8 @@ inline void tvp_powerup();
 
 inline void tvp_set_hpllcoast(alt_u8 pre, alt_u8 post);
 
+void tvp_set_coast_enable(alt_u8 enable);
+
 inline void tvp_set_linelen_tol(alt_u8 val);
 
 inline void tvp_set_ssthold(alt_u8 vsdetect_thold);