Rename rest to leftover

The length kind that asks for a part of what is left once the fixed
lengths are taken is called leftover: Len::leftover(2), Len::LEFTOVER,
Size::LEFTOVER, Len::leftover the field, and apply_leftover. It says
what it is where "rest" reads as "the remainder of the list" as often as
"the remaining space", and every agent who has touched this has reached
for a third word for it.

Locals called rest that meant a region or a widget are renamed with it,
since the word now names something else.

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

Cargo.toml

@@ -25,6 +25,9 @@ tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread", "time"]
 [workspace]
 members = ["core", "macro", "rig-input"]
 
+[profile.dev]
+debug = 1
+
 [workspace.package]
 version = "0.1.0"
 edition = "2024"