spec(043): mur find and the Reactor sample catalogue (#320)

Plans `mur find`/`mur get`/`mur list` as discovery-side tooling with
parity to winui-search: two-layer BM25 over a hand-curated catalogue of
~230 compilable scenarios (P0/P1/P2 enumerated), React-to-Reactor +
cross-framework synonym map, pitfall notes side-channel. Catalogue lives
at `samples/scenarios/<category>/<id>/{Scenario.cs, scenario.json}`;
build-time extractor validates Roslyn compilation as a CI gate. Ships
inside `mur.exe` (no separate binary). Supersedes `reactor-recipes`.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris