docs: describe current v2 loading state

Rewrite opencode-v2-compatibility.md as a current-state document:
registration rules (directory/package form, shared ~/.config/opencode/
config root, wrapper-dir pattern), dual-path loading on v1 hosts
(classic server() plus the embedded v2 pass whose [v2] failure logs
are expected noise), and the adapter/feature matrix without evolution
history. Sync the build description in src/v2/codemap.md.