docs: present mode and frame latency overrides

docs/development.md

@@ -120,6 +120,8 @@ By default PhotoCraft's own crates log at `info` and everything else at `warn`.
 | `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_PRESENT_MODE=mailbox` | Present mode for this launch: `fifo`, `mailbox`, `immediate`, `autovsync` or `autonovsync` (default: the platform's). For latency comparisons; shown in the stroke trace |
+| `PHOTOCRAFT_FRAME_LATENCY=1` | 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 `mailbox` the app no longer waits on vsync (`wait` near 0), and each refresh shows the newest frame. 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