perf: move runtime state files from SD card to tmpfs

[winstar0070]

Summary

• Move frequently-written runtime state files (now_fps, state, wifi_state, width, height) from /kvmapp/kvm/ to /tmp/kvm/ (tmpfs) to minimize SD card writes and extend SD card lifespan
• now_fps alone causes ~86,400+ writes/day (written every 3 seconds), significantly contributing to SD card wear
• User settings (fps, qlty, res, type) remain on /kvmapp/kvm/ as they need to persist across reboots

Related: #708, #701

Changes

File	Frequency	Change
now_fps	Every 3 seconds	/kvmapp/kvm/now_fps → /tmp/kvm/now_fps
state	On camera state change	/kvmapp/kvm/state → /tmp/kvm/state
wifi_state	On WiFi state change	/kvmapp/kvm/wifi_state → /tmp/kvm/wifi_state
width / height	On HDMI resolution change	/kvmapp/kvm/{width,height} → /tmp/kvm/{width,height}

Files modified (8 files)

• server/service/stream/frame_rate.go — now_fps write path
• server/service/network/wifi.go — WiFiStateFile constant
• support/sg2002/kvm_system/main/lib/system_init/system_init.cpp — add mkdir -p /tmp/kvm + update init paths
• support/sg2002/kvm_system/main/lib/system_state/system_state.cpp — update read/write paths
• support/sg2002/kvm_system/main/lib/hdmi/hdmi.cpp — update width/height write paths
• support/sg2002/additional/kvm/src/kvm_vision.cpp — update defines + state write paths
• support/sg2002/additional/sophgo-middleware/v2/sample/common/sample_common_sensor.c — update width/height read paths
• support/sg2002/kvm_system/main/include/config.h — update comments

Test plan

• Verify /tmp/kvm/ directory and files are created on boot
• Verify FPS value updates correctly via cat /tmp/kvm/now_fps
• Verify /tmp/kvm/width and /tmp/kvm/height update on HDMI connect/disconnect
• Verify /tmp/kvm/wifi_state transitions on WiFi connect/disconnect
• Verify /tmp/kvm/state changes on camera stream start/stop
• Verify user settings (fps, qlty, res, type) persist after reboot
• Verify /tmp is mounted as tmpfs: mount | grep tmpfs | grep /tmp