Correct docs/DEFAULTS.md against the real config engine, audit-style

Grepped the whole compositor for a second reader of every documented
general.*/theme.*/layout.*/performance.*/debug.*/platform.* config key
beyond its own seed call in crates/config/src/engine/support.rs.
Several sections turned out to be entirely or mostly decorative --
accepted, stored, sometimes range/type-validated, but never actually
applied to window/render behavior - which this file presented no
differently from the real, working settings around them:

- theme.colors.* (background/foreground/primary/secondary/accent/
  error/warning/success): none read past the seed call. The real,
  working theme surface is theme.decorations.* below it.
- performance.* and debug.* (vsync/max_fps/window_cache_size/
  event_queue_size/layout_timeout/enable_caching, logging/log_level/
  profile/trace_events/show_layout_bounds/show_window_geometry): every
  key in both namespaces is dead. Real frame pacing comes from the
  display's own hardware vsync; RUST_LOG is the real logging control.
- layout.tiling/dynamic/floating.*: only master_ratio is real
  (WindowManager::tiling.master_ratio). split_ratio, every behavior.*
  table, per-layout gaps.inner/outer, snap_threshold, grid_size,
  cascade_offset, smart_placement, default_position, remember_position,
  always_on_top are all dead - the real gap setting for every layout
  is general.window_gap.
- theme.decorations.border.focused_style/unfocused_style and
  title_bar.show/font: dead - solid is the only border style this
  compositor draws, and the titlebar always uses whatever system font
  find_system_font picks.
- platform.x11.*/platform.wayland.*/platform.windows.*/platform.macos.*
  use_*/global_hooks/accessibility_enabled: all dead - EWMH/NetWM/
  xdg-shell/layer-shell/DWM/Win32/Cocoa/Core Graphics support is simply
  always compiled in, never behind a toggle. platform.backend/
  platform.os are the two real, but read-only, keys in this section.

Also: the Environment Variables section listed SRDWM_THEME/
SRDWM_DEBUG_LEVEL/SRDWM_PLATFORM/SRDWM_MAX_FPS/SRDWM_VSYNC, none of
which exist anywhere in the codebase - replaced with the real set
(SRDWM_CONFIG_PATH, SRDWM_STATE_PATH, SRDWM_GPU, RUST_LOG), found by
grepping every std::env::var call in the compositor's own source.

Validation Rules' numeric list was missing corner-radius entirely
(theme.decorations.border.radius, 0-100, real in the validator despite
the doc gap) and resize_margin/inactive_dim, and didn't note that
srd set (unlike the Lua config path) doesn't run these checks at all.

Added general.gpu's own documentation section (new here - see
the matching commit adding the config option itself).

Also updates IMPLEMENTATION_STATUS.md's udev-backend paragraph, which
described the real GBM+EGL+DrmCompositor GPU path as not existing at
all ("that path needs a GPU...") - it now exists, opt-in, with its
current real scope (every head, cursor, no window content yet) noted
in place of the stale absence.

1 files changed, 11 insertions, 1 deletions

diff --git a/docs/IMPLEMENTATION_STATUS.md b/docs/IMPLEMENTATION_STATUS.md
index 43e2c5f..3365831 100644
--- a/docs/IMPLEMENTATION_STATUS.md
+++ b/docs/IMPLEMENTATION_STATUS.md
@@ -152,7 +152,17 @@ once, so splitting it across three files would have hidden it.
   GBM/EGL/`DrmCompositor`-based hardware acceleration: that path needs a
   GPU with working KMS+3D driver support that a low-spec machine's VM isn't
   guaranteed to have, while dumb buffers work on essentially any DRM
-  driver. `WaylandPlatform::connect` (winit) picks this backend
+  driver. A real GBM+EGL+`DrmCompositor` GPU path now exists
+  (`crates/wayland/src/udev/gpu.rs`), opt-in via `general.gpu` in config
+  or the lower-level `SRDWM_GPU=1` env var (both `false`/unset by
+  default - this backend stays 100% software unless explicitly asked
+  otherwise), falling back to the software path unchanged on any
+  failure at any step. Every connected head it successfully initializes
+  gets driven through it, VT-switch pause/activate is wired, and the
+  real cursor renders on top of its own clear color - window content
+  and decorations are the remaining gap: a GPU-driven head shows no
+  windows yet, only its clear color and cursor.
+  `WaylandPlatform::connect` (winit) picks this backend
   automatically when no `WAYLAND_DISPLAY`/`DISPLAY` is set, falling back to
   nested winit if udev init fails for any reason.
   **Verified live in an isolated QEMU VM** (see below): started on a bare