v0.76.0

Added

• A hub can now load its federation policy from an imported store at startup with
  synapse hub --federation-store FILE, so a peering imported with synapse federation import takes effect on the next start. The store's peerings — including revoked or expired
  ones, which authorise nothing — are composed into the live frame authorisation. Federation
  binds authority only alongside --require-message-auth; a store without it logs a warning
  that no cross-domain frame will be honoured, and a malformed store is reported and refused.
  With no store the live path is unchanged.
• Wired the federated trust policy into the live authorisation of agent frames, opt-in and
  deny-closed. A hub configured with a federation bundle now recognises a frame from a peered
  remote domain — identified only from its verified signing key and the live certificate pin,
  never a self-declared field — and authorises it against that peering's bounded scope, composed
  with mutual TLS, the event signature, and the mapped scope. A frame any layer refuses is
  refused with the reason named; a cross-domain frame on a hub that does not require per-message
  authentication is refused, since its authority cannot be bound. An allowed cross-domain frame
  is routed without the local access policy, which a remote subject has no identity in. A hub with
  no federation bundle is unchanged: every frame takes the local path exactly as before.
• Added a scope check that authorises a remote subject's frame against a peering's bounded
  scope, evaluated exactly as a local subject's frame is against the local access policy. Each
  access the frame requires is mapped to a verb in the remote subject's namespace, and every one
  must be granted by the peering's scope; a subject inherits no local default, so a frame with no
  granted verb, an empty scope, or no mapped access at all is denied rather than allowed. This
  keeps one authorisation vocabulary across local and cross-domain frames — only the policy they
  are evaluated against differs. Pure building block; not yet wired into the live frame path.
• Added a resolver that identifies which peered domain a frame belongs to from verified
  credentials alone. Given the Ed25519 signing-key id taken from a frame's verified signature and
  the certificate pin read off the live connection, it returns the single peered domain that
  accepts both, or nothing when no peering accepts both or more than one does. A key accepted by
  one domain presented over another domain's connection resolves to neither, and an ambiguous
  configuration is refused rather than guessed, so a frame's issuing domain is never taken from
  self-declared content. This is a pure building block; the live frame path is unchanged until it
  is wired in.