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.

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