- Enhance toolchain/parse/node_kind.def with category x-macros
(CARBON_PARSE_NODE_KIND_DECLARATION, CARBON_PARSE_NODE_KIND_EXPRESSION,
CARBON_PARSE_NODE_KIND_PATTERN, CARBON_PARSE_NODE_KIND_STATEMENT) that
default to CARBON_PARSE_NODE_KIND, keeping node_kind.def as the single
source of truth without manual expansion divergence.
- Decompose the monolithic toolchain/parse/extract.cpp translation unit
into separate compilation units for declarations, expressions, patterns,
and statements by expanding respective category x-macros.
- Forward-declare Tree and TokenizedBuffer in parse and lex dump headers
to reduce header inclusion depth.
Assisted-by: Antigravity with Gemini
