chore: stale kind-range docs and a deprecated manifest key

What

Two pieces of maintenance:

The kind: doc lines in src/context.mach (around lines 3407 to 3534) give the range as theme.LABEL .. theme.SCRIM, but built-in kinds now run past SCRIM (FOCUS_RING, and SLIDER_FILL from feat: slider value fill #167). They should name the range in a way that does not go stale when a kind is added.
Every manifest (mach.toml and each demo's) sets default = true under [target.linux-x86_64], which mach now warns is deprecated and has no effect.

The kind-range docs are correct and do not name the last kind
No manifest sets the deprecated key, and building the root and each demo prints no warning from the manifests

docs(#181): do not name the last theme kind in kind docs

chore(#181): drop deprecated default key from target tables

chore(#181): drop the panel demo's unused bool import

Merge pull request #188 from briar-systems/chore/181