feat(editor): add snippet library, tab-stop engine, and $ palette

Parse the Zed-compatible .typoena.snippets.json (serde_json), strip
${n:label} stops to bare $n, and drive a forward-only tab-stop session.
Expand a prefix inline on Tab in Insert; browse and insert from the
Cmd-P palette via a $ sigil (parallel to the > command mode), landing
on $1 as one undo group.

editor/Cargo.toml

@@ -8,3 +8,8 @@ description = "Modal (vim-style) text-editor core: the buffer, motions, edits, a
 embedded-graphics = "0.8"
 display = { path = "../display" }
 keymap = { path = "../keymap" }
+# Snippet library parse (v0.6): the `.typoena.snippets.json` file uses Zed's JSON
+# shape. serde_json handles the string-escape corner cases a hand-rolled reader
+# would get wrong; the crate is `std` (esp-idf provides std on xtensa).
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"