Fix spawn placement under the top bar, add per-window minimum sizes, and clean up maximize

Four reports after restarting into today's build, with a screenshot. The
screenshot was measured, not eyeballed: top bar y=0..29, a 4px accent border
at y=30..33, titlebar from y=34, bottom border at y=1027..1030, and 49px of
bare desktop below it.

Windows spawning too close to the top bar. A remembered position was
validated only by asking whether it landed on some monitor's full_geometry,
which includes the strip a top bar reserves, so an app whose remembered y was
small reopened with its titlebar under the bar. That is why it was
"sometimes": it depended on the stored value, and the live store holds
wezterm at y=44 and firefox at y=69 against a 30px bar. Remembered positions
are now clamped into the monitor's usable area.

Placement not surviving a logout. Window memory does persist, but five of the
eleven entries in the live store were saved with a second monitor attached,
at x >= 2000. Those points match no current monitor and were discarded
outright, falling back to a fresh cascade, so those apps appeared to remember
nothing. Such a position is now clamped onto a monitor that exists instead.

Per-window minimum sizes. One global floor is wrong in both directions.
Three sources now, in increasing precedence: the global floor, the client's
own declared minimum (xdg_toplevel.set_min_size or ICCCM hints), and a
min_width/min_height window rule overriding both. A rule wins permanently --
the backend refreshes the client's declared minimum on every decoration
redraw and must not undo a deliberate override.

Maximize, three faults in one report. A maximized window now draws no
border: its edges are the screen's edges, and the only place maximize stops
short is the bar strip, which is exactly where the measured line was.
maximize_geometry_for no longer subtracts a bottom-anchored dock's exclusive
zone, so maximize runs to the bottom of the screen and the dock floats over
it; top, left and right are still honoured.
general.maximize_covers_dock = false restores the old behaviour. With the
border gone the window sits flush under the bar instead of with an accent
line crowding it.

Verified: seven new tests on the real numbers from the live store, and
maximize geometry measured live in a nested instance (a window maximized on a
split half reports exactly that half's rect). NOT confirmed on screen: the
border removal and the dock behaviour - the nested backend has no bar or
dock to reserve a zone, and an attempt to check the border produced a failing
control, since srd set border_width only affects windows created after it.

515 tests pass, clippy clean.

1 files changed, 54 insertions, 0 deletions

diff --git a/docs/DEFAULTS.md b/docs/DEFAULTS.md
index 3497410..2cc1614 100644
--- a/docs/DEFAULTS.md
+++ b/docs/DEFAULTS.md
@@ -20,6 +20,7 @@ srd.set("general.gpu", false)                          -- Default: false - udev
 srd.set("general.desktop_icons", true)                 -- Default: true - see "Desktop icons" below
 srd.set("general.desktop_icons_all_monitors", true)    -- Default: true - mirror icons onto every monitor, not just primary
 srd.set("general.config_reload_on_write", true)         -- Default: true - re-read init.lua when it changes on disk
+srd.set("general.maximize_covers_dock", true)          -- Default: true - maximize runs under a bottom dock, not above it
 srd.set("general.reserve_top", 0)                      -- Default: 0 - static space reserved before any bar/dock connects
 srd.set("general.reserve_bottom", 0)                   -- Default: 0 - see "Startup space reservation" below
 srd.set("general.reserve_left", 0)                     -- Default: 0
@@ -881,3 +882,56 @@ One limit to know: a reload does not re-register key *grabs* with the
 backend, so a brand new key combination needs a restart before the
 compositor sees that key at all. An existing combination picks up its new
 action immediately.
+
+## Window minimum sizes
+
+Every window has a minimum size it cannot be resized below. Three sources,
+in increasing precedence:
+
+1. A global floor, used when nothing else applies.
+2. The client's own declared minimum - `xdg_toplevel.set_min_size` for a
+   native Wayland window, the ICCCM size hints for an XWayland one. Read
+   from the client and refreshed as it changes.
+3. A `min_width`/`min_height` window rule, which overrides both. Either may
+   be given alone; the other axis keeps the global floor.
+
+```lua
+srd.rule({ class = "Alacritty" }, { min_width = 480, min_height = 320 })
+```
+
+A rule wins permanently: the client's own declared minimum is never allowed
+to overwrite it, so an application that reports a minimum too large for a
+small screen can be overridden. The minimum applies to interactive resizing,
+to a size restored from window memory, and to any programmatic resize.
+
+## Maximize
+
+Maximize targets the monitor's full rectangle minus the space reserved by
+top, left and right layer-shell surfaces. A bottom-anchored dock is
+deliberately not subtracted, so a maximized window runs to the bottom of the
+screen and the dock floats over it - the usual behaviour for an auto-hiding
+dock. Set `general.maximize_covers_dock = false` to have maximize stop above
+the dock instead.
+
+A maximized window draws no border. Its edges are the screen's edges, so a
+border has nothing to separate it from, and where maximize does stop short
+(the strip a top bar reserves) a border would otherwise draw as a hard line
+directly against the bar.
+
+## Where a window opens
+
+In order: a `geometry` window rule wins outright; otherwise a position and
+size remembered for that `app_id` are restored; otherwise smart placement
+picks a free grid cell, falling back to a cascade.
+
+A remembered position is clamped into the target monitor's *usable* area --
+the region left after bars and docks reserve their space. Without that
+clamp, an app whose remembered `y` was smaller than the top bar's height
+reopened with its titlebar underneath the bar, where it could not be
+grabbed. A position remembered on a monitor that is no longer connected is
+clamped onto a current monitor rather than discarded, so undocking does not
+make an app forget where it lives.
+
+Dialogs are centred instead, and take neither a remembered position nor a
+remembered size - window memory is keyed by `app_id`, which a dialog shares
+with the window that spawned it.