docs: capture sensor-side WDR via /mnt/mtd/Config/wdrmode.dat on Sofia

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Documents the procedure for capturing sensor-side WDR mode init from a XiongMai Sofia build, after the multi-step investigation that turned up several false leads.

TL;DR of the new section

Sofia's runtime DVR-IP BroadTrends.AutoGain knob does not reconfigure the sensor — it lands in Public_LinearWDR_Switch → LibXmCap_System_switchWdrMode → XmCap_IspVi_switchWdrMode which only reads the ISP attr (the misleading HI_MPI_ISP_SetWDRMode failed error string is a copy-paste relic from a refactor) and persists the new mode to /mnt/mtd/Config/wdrmode.dat via libdvr.so's SetWdrMode (a thin fopen + fwrite of a 4-byte LE int). The sensor-side mode dispatch happens at the next Sofia startup: Sofia reads wdrmode.dat and calls cmos_set_image_mode(mode) inside the per-sensor driver, which then runs the alternative init register table.

So the actual procedure is:

printf '\x02\x00\x00\x00' > /mnt/mtd/Config/wdrmode.dat ; sync   # mode 2 = line-WDR
killall Sofia ; sleep 1
WDR_TRACE=/utils/dumps/mode2.log /usr/bin/Sofia </dev/null \
    > /utils/dumps/mode2-stdout.log 2>&1 &                       # via bind-mount wrapper
sleep 30 ; killall ipctool-upx Sofia

Then verify the right mode fired by grep'ing Sofia's stdout for the per-mode banner (linear mode / 2to1 line WDR ... / LINE Init OK).

Worked example: IMX291 + CV300 + Sofia

metric	mode 0 (linear)	mode 2 (line-WDR)
trace size	20 KB	51 KB
total writes	70	140
Sofia banner	linear mode	2to1 half frame WDR mode + 60fps LINE Init OK
segmenter phases	init(106)	init(106) + mode_switch_1(114)

WDR-only registers: 0x3007=0x00, 0x3009=0x02, 0x3018=0x6D, 0x3046=0xE1, 0x300A=0xF0, 0x3014=0x20, 0x3020/0x3021 — Sony's IMX29x DOL configuration block.

Tooling unchanged

trace_segment.py's existing find_mode_switches heuristic (added in #149, generalised in #152) catches Sofia's two-pass init as init + mode_switch_1 with no code changes — works because Sofia's cmos_set_image_mode per-mode init wraps each register table between a standby-assert/release pair on the family's stream-control register, which is exactly what the heuristic looks for.

trace_to_driver.py emits the WDR body as <sensor>_set_mode_1 next to <sensor>_linear_init, all gcc -Wall -Wextra clean.

Diagnostic added

The new section ends with a recipe for answering "does my sensor have WDR firmware in this Sofia build?":

$ strings /usr/bin/Sofia | grep -E '_init  |LINE Init OK' | sort -u
~~~~~~~~~~AR0237_init  JJ ~~~~~~~~~          ← linear-only
--IMX291 1080P 60fps LINE Init OK!           ← has WDR
--IMX290 1080P 30fps LINE Init OK!           ← has WDR
~~~~~~~~~~IMX323_init  gjj ~~~~~~~~~         ← linear-only

A sensor with only ~~~_init~~~ markers is linear-only on that build; wdrmode.dat writes still get persisted but produce no sensor-side effect.

Also generalises the Segmenter heuristic note

The existing paragraph said find_mode_switches watches for 0x100=0 → 0x100=1. That was true only for SmartSens; #152 made the function take whichever (reg, init_val, stream_val) tuple find_init_bounds matched on. The note now reads "the same stream-control register pair that the matched init_pattern used", with explicit examples for SmartSens / Sony IMX / SOI/JX.

Test plan

• tools/test_pipeline.sh (CI smoke test) — passes
• No tooling changes — purely docs

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