Concept-compatible public API, not drop-in compatibility

modern-spf re-creates SPF's concepts and recognizable surface — spf.init / spf.navigate / spf.prefetch, spf* events, the spf-link class — but modernizes the semantics (async/await, Promise-returning calls, typed events, streaming responses). We deliberately do not guarantee that old SPF sites or servers work unchanged: drop-in fidelity would force the legacy callback-based, string-typed API onto the public contract, which is the part we set out to modernize.

• Status: accepted
• Considered Options: (a) full drop-in — locks in legacy semantics; (b) concept-compatible — chosen; (c) inspired-only — throws away the recognizable surface this project is named for.
• Consequences: server code needs small tweaks (e.g. Accept-header negotiation); the ?spf= identifier and JSON field names are kept so the changes stay small.