Update the peer README and architecture notes to match the landed runtime:
version.ini is the download sentinel, local/ is the install predicate, install
state is recovered through .lanspread.json intents, and watcher rescans are
operation-gated rather than time-debounced.
Add IMPL_DECISIONS.md with the implementation-time choices that were not
already prescribed by PLAN.md, including the just test recipe, the UI event
compatibility bridge, reuse of the existing library index for per-ID rescans,
and the split between active operation state and download cancellation tokens.
Test Plan:
- just fmt
- just clippy
- just test
- just build
Refs: PLAN.md
