Bind Self to the conformance target; add ce.conformsTo — Fix #362

[enumeratio]

Fix #362

• Self in a protocol member's signature now binds to the conformance target that dispatch selects for the receiver (the most specific applicable edge, as bestCandidates picks it), not the receiver's own static type. With Compare: "(Self, Self) -> number" on real, Compare(5, 1/2) and Compare(3, 2.5) were rejected while the reverse orders passed; all four now pass, both at canonicalization and at run time, and a -> Self result binds the same way. Arguments are still checked against the target, not a join of their types, so a string argument or a non-conforming nominal type is still refused. Compile already grounded Self at the target, so it's unchanged. Appendix A of TYPE_SYSTEM_ROADMAP.md is updated.
• ce.conformsTo(type, protocol) is the public counterpart of the resolver's internal conformsTo, including conditional conformances. It sits next to declareProtocol, and api.md is regenerated.

Protocol refinement is left as a separate exploration, as you suggested.

[enumeratio]

Pushed a review pass:

• Added compiled-lane tests for the mixed-subtype case to protocol-dispatch-compile.test.ts: difference(5, 1/2), difference(1/2, 5) and the float pairs compile and agree with .N(). Main fails the two orders where the receiver is the narrower type.
• Rewrote the new comments and the conformsTo doc to state the current rule, without the history or bare design references, and regenerated api.md.

This PR doesn't slow member calls on its own. Combined with #365 they stay at #365's cost.