PayloadMode.SEMANTIC_GRAPH.is_implemented returns False, blocks Mode 3 negotiation

[phillipclapham]

Problem

In ldp-protocol, PayloadMode.SEMANTIC_GRAPH exists in the enum but is_implemented returns False. This means negotiate_payload_mode() will never select Mode 3, even when both parties declare support for it.

Any client using negotiate_payload_mode() directly (which is the expected path) can't discover or negotiate Mode 3 capability.

Current workaround

In flowscript-ldp, our FlowScriptMode3Delegate overrides _handle_session_propose() to bypass the is_implemented gate entirely. The delegate knows it IS the implementation, so it checks the initiator's supported modes directly and negotiates SEMANTIC_GRAPH when both sides support it.

This works, but it means Mode 3 negotiation only succeeds when our delegate handles the session proposal. Any generic LDP client or router that calls negotiate_payload_mode() won't see Mode 3 as an option.

Proposed approaches

Option A: Flip the flag. Set is_implemented = True for SEMANTIC_GRAPH now that a reference implementation exists. Simple, but arguably premature if the spec wants to gate on multiple implementations or a conformance test.

Option B: Registration mechanism. Let implementations declare support at runtime:

PayloadMode.register_implementation(PayloadMode.SEMANTIC_GRAPH, "flowscript-ldp")

This keeps is_implemented accurate to the local environment. It returns True only when something is actually installed that handles it. More work, but more correct.

Option C: Remove the gate from negotiation. Let negotiate_payload_mode() consider all modes that both parties declare, regardless of is_implemented. Implementations that can't actually handle a mode simply shouldn't declare it. The is_implemented flag becomes informational rather than a hard gate.

I'd lean toward B or C, but happy to go with whatever fits the SDK's design direction. Can submit a PR for whichever approach you prefer.