[marcoallegretti]

Fixes #164.

Store session logs under $XDG_STATE_HOME/shoji_wm/logs/, falling back to
~/.local/state/shoji_wm/logs/. Ignore empty or relative XDG_STATE_HOME
values. Add SHOJI_LOG_DIR to override the complete directory; empty
overrides are ignored.

Timescope profiles use the same resolver. Rotation and retention remain
unchanged and operate within the selected directory.

The default-path change does not migrate or delete files under
~/shoji_wm/logs/. Selecting that directory with SHOJI_LOG_DIR preserves
the old location, with normal rotation and retention applying there.

Update English/Japanese documentation and the profiler SDK comment.

API impact:

• No Rust or TypeScript signature changes.
• New environment option: SHOJI_LOG_DIR.
• The default log/profile directory changes.
• No new CLI option or dependency.

Validation:

• The original path-resolution regression test failed against the previous
  behavior.
• cargo test -p shojiwm_lib --lib --locked log_path_tests:
  3 parent tests and 13 child executions passed.
  These verify environment precedence, flushed log contents, real Timescope
  reports containing a recorded sample, profile rotation, log rotation,
  overwrite mode, byte/age retention and preservation of legacy files.
• Rebuilt shoji_wm and ran three nested Wayland sessions under headless
  Weston with software rendering: XDG, HOME fallback and explicit override.
  Each loaded a minimal TypeScript configuration, answered a Wayland
  registry/sync request, acknowledged quit and exited with status 0.
  Session logs and shutdown profiler reports were verified at each path.
• npm run typecheck: passed.
• npm --prefix docs run build: English and Japanese builds passed.
• Rust formatting and git diff --check: passed.
• Full library suite: 315 passed, 2 failed, 4 ignored.
  Both failures are unchanged Xwayland socket tests that cannot prepare
  /tmp/.X11-unix because it is not writable.

The existing Scene3D warnings are addressed separately by #167.
This branch starts at main and does not depend on #167 or #168.

Nested validation covers software Wayland with a minimal configuration,
not hardware DRM presentation or visual correctness.