Phase 3b of the shell theme system. A slot list entry can now be
`widget:<key>`, where `<key>` is either a WidgetPlacement alias
(right_of_workspaces, left_of_clock, right_of_clock, left_of_stats, tray)
or a Lua module name. ModuleRegistry::for_each_in_slot creates each
widget container on demand at its slot position; reconcile_widgets routes
each WidgetSpec by module name first, falling back to its placement
alias, and logs+drops (never panics) a spec with no matching container.
bread_shared::widget::WidgetPlacement stays untouched — it's still the
wire type breadd sends, unmodified and unshadowed; only the container
map that placement now resolves through is theme-driven.
Adds ModuleRegistry (src/bar/slots.rs), mapping each [bar.slots] module
name (workspaces/media/clock/volume/wifi/battery/control) to its
already-built widget. main.rs now registers the seven modules once
they're constructed, then walks ShellTheme::slots() to append them
into the left/centre/right containers in theme order instead of a
hardcoded sequence. An unknown module name in a theme manifest is
logged and skipped rather than panicking.
The Lua-declared widget_* containers keep their fixed interleave
(right-of-workspaces, left/right-of-clock, left-of-stats) — that's
Phase 3b, not this change.
