Desktop: write log records to standard error and a rotating log file

The desktop app installed no logger, so every log::warn!/log::error! in
the workspace was discarded: the graphics adapter in use, a lost graphics
device, the engine guard's report of a recovered panic. A launch from a
desktop menu has no terminal either, so there was nothing to attach to a
bug report.

apps/vectorcraft/src/logging.rs (no new dependency):
- standard error plus logs/vectorcraft.log next to the preferences
  (~/.config/vectorcraft/logs, or under $XDG_CONFIG_HOME); each launch
  rotates the previous log to vectorcraft.1.log and .2.log, so the log of
  a run that crashed survives the next start; the file is capped at 16 MiB
- records logged before the settings directory is known are buffered
  (bounded) and written once the file is attached
- default levels: info for vectorcraft*, warn for everything else;
  RUST_LOG replaces them with env_logger-style directives
- never panics: an unusable directory or a failed write leaves standard
  error as the only sink
- runs with VECTORCRAFT_NO_PREFS (agents' test runs) log to standard
  error only, so they don't rotate away the user's own logs

The logger is installed first in main and the file is opened after
argument parsing (--version leaves no file). Log messages of this crate
drop their "vectorcraft: " prefix, which the record's target now shows.
vectorcraft-cli keeps its own MCP logger (vectorcraft_mcp::logging).
Documented in docs/development.md (Logs, Environment variables).

docs/development.md

@@ -42,6 +42,21 @@ The canvas is rasterized on the CPU (`vectorcraft-render`, vello_cpu); the GPU (
 - The `WGPU_POWER_PREF` environment variable (`low`, `high` or `none`) overrides the preference.
 - Help › About and the control channel's `ui.inspect` (`graphicsAdapter`) show the adapter in use, and the app logs it at startup.
 
+## Logs
+
+The desktop app writes its `log` records to standard error and to `logs/vectorcraft.log` next to the preferences (Linux `$XDG_CONFIG_HOME/vectorcraft/logs/`, by default `~/.config/vectorcraft/logs/`; macOS `~/Library/Application Support/VectorCraft/logs/`; Windows `%APPDATA%\VectorCraft\logs\`). A start launched from a desktop menu or the Dock has no terminal, so this file is what to attach to a bug report: the graphics adapter in use, a lost graphics device, panics the engine's guard recovered from and clipboard formats that couldn't be made land there. Each launch moves the previous log to `vectorcraft.1.log` (and that one to `vectorcraft.2.log`), so the log of a run that crashed survives the next start. The file stops growing at 16 MiB. `--version` writes no file, and runs with `VECTORCRAFT_NO_PREFS` log to standard error only.
+
+By default VectorCraft's own crates log at `info` and everything else at `warn`. `RUST_LOG` replaces that with env_logger-style directives, for example `RUST_LOG=debug`, `RUST_LOG=warn,vectorcraft_render=trace` or `RUST_LOG=info,wgpu_core=warn`; a directive ending in `*` covers every target starting with it (`vectorcraft*=debug`). The logger is `apps/vectorcraft/src/logging.rs`. `vectorcraft-cli mcp` has its own logger, which sends records to the MCP client (`docs/mcp.md`).
+
+## Environment variables (desktop app)
+
+| Variable | Effect |
+|---|---|
+| `VECTORCRAFT_CONTROL_PORT` | Same as `--control <port>` |
+| `VECTORCRAFT_NO_PREFS` | No preferences read or written, no default Data Recovery folder and no log file (agents' test runs) |
+| `WGPU_POWER_PREF` | Graphics adapter: `low`, `high` or `none` (see [Desktop graphics processor](#desktop-graphics-processor)) |
+| `RUST_LOG` | Log levels for standard error and the log file (see [Logs](#logs)) |
+
 ## Linux: Wayland and X11
 
 The window runs natively on Wayland (eframe's `wayland` feature) and on X11. The system clipboard (`apps/vectorcraft/src/clipboard.rs`) is arboard with its `wayland-data-control` feature: