docs(rfc): Swift-native transition strategy (repo-internal, not on the docs site)

rfcs/0001: goals and hard constraints (deps from apple/swiftlang/
pointfreeco only, vetted-exception process with Vapor as the named P6
candidate, system sqlite3/zstd/harfbuzz allowed), full current-state
inventory, the bun:ffi + @_cdecl bridge with a per-module
APPLE_DOCS_NATIVE kill switch and CI parity gates (golden NDCG/MRR,
benchmarks, snapshot determinism), pinned toolchains with static-musl
Linux cross-compilation, and phases P0–P7 from leaf hot functions and
the from-scratch model2vec embedder through renderers, content,
storage, servers, and the final single static binary. Lives in rfcs/
outside the VitePress srcDir so the production docs site never builds
or indexes it; pointer added to ARCHITECTURE.md.