Add fixed-refresh preset group for workstation modes

get_pure_lm_mode() picks the preset with the nearest total line count.
Several workstation display modes share line counts with each other or with
VESA modes while having one fixed refresh rate (e.g. Sun 1152x900 is 937
lines at both 66 Hz and 76.6 Hz on cg6 boards, and Sun 1280x1024@76 has the
1066 lines of VESA 1280x1024).

Add GROUP_FIXED_VHZ. Presets in this group are candidates only when the
measured refresh is within +-0.3 Hz of the preset (the measured refresh is
exact to about 0.01 Hz). They are appended at the end of the table, so the
early exit of the search loop is replaced by a flag that skips the
remaining regular presets exactly as before but still lets fixed-refresh
presets take part, including the existing equal-line refresh tie-break
(group >= GROUP_720P). Group-indexed arrays in video_modes.c and
controls.c get the extra entry.

No preset uses the group yet, so behaviour is unchanged.

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

software/sys_controller/inc/video_modes.h

@@ -70,8 +70,11 @@ typedef enum {
     GROUP_720P      = 8,
     GROUP_1080I     = 9,
     GROUP_1080P     = 10,
+    GROUP_FIXED_VHZ = 11,   // fixed-refresh workstation modes: only matched when the input refresh is within FIXED_VHZ_TOL_X100
 } video_group;
 
+#define FIXED_VHZ_TOL_X100  30      // +-0.30 Hz (measured refresh is exact to ~0.01 Hz)
+
 typedef enum {
     MODE_INTERLACED     = (1<<0), //deprecated
     MODE_CRT            = (1<<1),