docs: add implementation plan for the Lua dashboard

Ten tasks, TDD where there is logic to test. Ordered so nothing
half-built touches the working desktop: pure Lua and its tests first,
then pixels, then the palette and the widget, then window placement,
and only last does UDT switch over to this repo's template.

Leads with the platform facts verified during design, because each one
invalidates the approach an implementer would otherwise reach for:
conky_surface() instead of the Xlib idiom on Wayland, own_window_type
normal rather than desktop, a Lua fault presenting as a blank screen,
grim capturing screen coordinates so it needs the dashboard's workspace
active, and Cairo having no Black font weight.

The pcall error overlay is built before any widget and verified by
injecting a deliberate fault, since it is the only channel that makes a
later mistake visible.

Fixture arithmetic in the parser tests is checked rather than asserted
from memory: a wrong expected value sends the implementer chasing a bug
that is not there.

Paths are ~-relative throughout, so the plan does not pin to one home
directory.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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