tokens is a required parameter on _flag_defines and _project_flags now,
so the per-consumer re-lex the shared lexer was meant to end cannot
come back. The cache stamp covers the gcc binary's size and mtime via
a shared _stat_sig (an in-place toolchain re-extraction regenerates
the script like a header edit). A bare -I or -D from an empty argument
is warned and skipped instead of letting gcc eat the next flag.
_defines_flags documents that its tokens carry shell-level escaping and
must be emitted unquoted, and _MMU_DEFAULT notes why the board-manifest
values are hardcoded.