Low-Latency Drawing preference (vsync off, opt-in), plus present-mode overrides for testers (#1343) (#5274)

* PHOTOCRAFT_PRESENT_MODE / PHOTOCRAFT_FRAME_LATENCY launch overrides for latency A/B tests (#1343)

Linux traces put the remaining stroke latency in presentation queueing (the app's frame fits
in one refresh; the rest waits on the swap chain), and an NVIDIA user cured it by forcing
vsync off. Testers can now compare without rebuilding: PHOTOCRAFT_PRESENT_MODE (fifo,
mailbox, immediate, autovsync, autonovsync) and PHOTOCRAFT_FRAME_LATENCY (1-3), applied over
the platform default. Bad values are logged and ignored. The active mode is logged at start,
kept in GpuInfo.present and shown in every stroke trace line.

RTX 5090, Vulkan, 120 Hz: default autovsync/latency 1 waits 6 ms per 8.3 ms frame; mailbox
renders every 2.3 ms with 0.2 ms wait, so each vsync shows the newest frame.

* docs: present mode and frame latency overrides

* Present mode overrides never request a mode the driver may lack (Mailbox crashed on X11/NVIDIA)

* Preferences › Performance › Low-Latency Drawing, on by default: no vsync wait while drawing (#1343)

Testers' traces put the ~50 ms stroke lag in the swap chain's vsync wait. With vsync off it
disappears (X11/NVIDIA; Windows/NVIDIA via the driver), and a shorter queue doesn't help. The
surface now presents with AutoNoVsync by default: wgpu picks Immediate, else Mailbox, else
Fifo by what the driver offers, so no unsupported mode is ever requested. Turning the
preference off brings vsync back (smooth, but late). It applies at the next launch, like the
other graphics settings. Where only Immediate exists the canvas may tear while it changes;
frames render as fast as input arrives during a stroke and stop when idle.

* Runtime and launch controls: ui.set strokeTrace, --stroke-trace, --present-mode, --frame-latency

The retro's remaining gap: the stroke trace and the presentation overrides were env-only. The trace now switches at runtime over the control channel (ui.set {"strokeTrace": true|false|"stderr"|path}) and with --stroke-trace; --present-mode / --frame-latency sit beside their env vars and win over them. Bad values are reported and ignored; none fails a launch.

* docs: strokeTrace beside the other ui.set fields

* docs: regenerate the scorecard after rebasing onto main

* Low-Latency Drawing: frames capped at 240 fps without vsync (after #4472); ui.set strokeTrace takes on/off only (a CSV path is launch-only); tearing documented as usual (Immediate); pref hidden on web; doc comment order

* stroke_trace: from_value doc says ui.set passes only on or off

* Low-Latency Drawing ships off (opt-in), per the owner's decision

* docs: present mode default is vsync (Low-Latency Drawing ships off)

docs/development.md

@@ -119,7 +119,9 @@ By default PhotoCraft's own crates log at `info` and everything else at `warn`.
 | `PHOTOCRAFT_CPU_COMPOSE=1` | Keep the wgpu canvas but composite on the CPU (compare GPU vs CPU renders, e.g. with the snapshot example) |
 | `PHOTOCRAFT_LOCALE` | Override Auto's native UI language for this launch; unsupported tags use English. See [system language detection](localization.md#first-launch-and-system-language). |
 | `PHOTOCRAFT_FX_TRACE=1` | Print the CPU time spent on GPU effect shapes and distance fields per rebuild |
-| `PHOTOCRAFT_STROKE_TRACE=1` (or `=<file.csv>`) | Time every frame of each brush stroke; print a summary per stroke, and with a path also append each frame to that CSV (see [Stroke latency trace](#stroke-latency-trace)) |
+| `PHOTOCRAFT_STROKE_TRACE=1` (or `=<file.csv>`; also `--stroke-trace <stderr|file.csv>`, or `ui.set {"strokeTrace": true|false}` at runtime, without a file) | Time every frame of each brush stroke; print a summary per stroke, and with a path also append each frame to that CSV (see [Stroke latency trace](#stroke-latency-trace)) |
+| `PHOTOCRAFT_PRESENT_MODE=novsync` (or `--present-mode`) | Present mode for this launch: `fifo`, `autovsync`, or `novsync` (also `immediate`, `mailbox`: no vsync, the first of Immediate, Mailbox, Fifo the driver offers). Default: vsync, or no vsync when Preferences › Performance › Low-Latency Drawing is on (it ships off). For latency comparisons; shown in the stroke trace |
+| `PHOTOCRAFT_FRAME_LATENCY=1` (or `--frame-latency`) | Frames the swap chain may queue, 1 to 3 (wgpu's GL backend ignores it). For latency comparisons; shown in the stroke trace |
 | `PHOTOCRAFT_THEME_FILE=tokens.json` | **Debug builds only:** live design-token overrides, re-read on change |
 
 ### Live design tokens
@@ -148,7 +150,7 @@ stroke trace: 62 frames on vulkan NVIDIA GeForce RTX 5090 (p50/p95 ms): interval
 | `cpu` | eframe's CPU time per frame: UI, tessellation and GPU submission, without the vsync wait |
 | `wait` | `interval − cpu`: waiting on vsync or the swap chain, or idle |
 
-What the app can't time is the rest of input-to-photon: the OS input queue before a frame starts (up to one `interval`), frames queued for the display (the surface keeps up to two; the GL backend ignores that setting), and the compositor. So the first line names the backend. The last summary is also in the control channel's `perf.strokeTrace` (`ui.inspect`). The code is `crates/ui-egui/src/stroke_trace.rs`.
+What the app can't time is the rest of input-to-photon: the OS input queue before a frame starts (up to one `interval`), frames queued for the display (the surface keeps up to two; the GL backend ignores that setting), and the compositor. So the first line names the backend and how frames are presented. To compare presentation settings without rebuilding, launch with `PHOTOCRAFT_PRESENT_MODE` and `PHOTOCRAFT_FRAME_LATENCY` (see [Environment variables](#environment-variables)): with `novsync` the app no longer waits on vsync (`wait` near 0): each refresh shows the newest frame, and where the driver only offers Immediate, the screen may tear. The last summary is also in the control channel's `perf.strokeTrace` (`ui.inspect`). The code is `crates/ui-egui/src/stroke_trace.rs`.
 
 ## Driving the app programmatically