sdk: add guest seams and mocks for identity, messaging and remote-store

[mfw78]

Add guest SDK seams and mocks for the identity, messaging and remote-store host interfaces, and wire them to the stub backends. These three of the six host interfaces currently have no guest seam.

Why

The identity, messaging and remote-store interfaces carry adapter:None in the macro known table, have no *Host trait and no Mock*, so the promise that each interface becomes a trait plus a mock is unfulfilled for all three. Without them, modules cannot unit-test host-free against these interfaces. Part of milestone M3: Videre SDK, macros and DX. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Add IdentityHost, MessagingHost and RemoteStoreHost guest traits plus a Mock* for each.
• Make the bind macros recognise the three new traits and wire the corresponding bind-macro slices.
• Wire the three seams to the existing stub backends, with no change to backend liveness scope.
• Widen the Host supertrait to cover all six interfaces, or document opt-in subset supertraits.

Done when

• IdentityHost, MessagingHost and RemoteStoreHost guest traits plus their Mock* exist and are recognised by the bind macros.
• The three seams are wired to the stub backends.
• The Host supertrait covers all six interfaces, or the subset supertraits are documented.
• Modules can host-free unit-test against all three interfaces.

[mfw78]

Landed via #455 (squash-merged into dev/m1 as 709d189). Closing manually since the merge target is not the default branch.