Changelog: record the parser and the known gaps

The packaged change-notes still described only the lexer and highlighting, so an
installed build understated what it does and said nothing about the 58 corpus
files that still fail.

CHANGELOG.md

@@ -11,3 +11,14 @@
   including nesting block comments and here-strings
 - Syntax highlighting and a colour settings page
 - Comment/uncomment support and brace matching, including `.{` and `.[`
+- Grammar-Kit parser and PSI for the whole language: declarations, procedures,
+  structs, enums, unions, control flow including the `if x == { case }` switch
+  form, and compiler directives. 91.9% of the 714-file Jai distribution parses
+  with no syntax errors.
+
+### Known gaps
+
+- 58 files in the Jai distribution still report a syntax error; each involves a
+  construct used in only one or two files. `#asm` bodies are parsed as opaque
+  blocks.
+- Structure view, folding, and go-to-definition are not implemented yet.