The tree is Ubuntu-first, but the Linux-specific surface is about ten files and
already sits at the bottom of the I/O and RPC stacks. Rather than fork the repo
or scatter ifdefs, platform differences go behind narrow seams with one backend
directory per OS, selected centrally in cmake/platform.cmake.
Linux stays the reference implementation and its behaviour does not change: the
owning lanes (CORE, DAEMON) move today's Linux code behind the seams on Ubuntu
first, with the suite still green, before any macOS code exists. A new PORT lane
owns only the per-OS backends, so macOS work never writes shared logic and
CLAUDE.md's lane rule stays intact.
Adds ADR 0020 (the decision and phases), docs/08-porting.md (seam list, API
mapping for macOS and Windows, dependency table, verification gates) and
docs/agents/AGENT-PORT.md (the lane brief), plus the PORT row in the ownership
table.
Co-Authored-By: Claude Opus 5 <[email protected]>
