README explains what the rewrite covers, how to build, run, install and
test it. ARCHITECTURE.md gains a top-down decomposition tree (binary ->
runtime/core -> termio/terminal/renderer -> leaf crates) and an updated
crate map with the actual internal dependencies, including the crates
added during integration (apprt, inspector, onig, bench, build-data).
CHANGELOG records the rewrite under Unreleased; no version bump.
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