Add generic information-flow control core (#7293)

Adds a dependency-free `ifc-core` crate implementing reader-set
confidentiality labels and monotonic per-computation flow state.

- Defines flow ordering, join, and meet over caller-supplied principal
universes.
- Fails closed for unknown or cross-universe input and checks reader
widening at egress.
- Includes property tests for lattice laws and monotonic taint, plus the
design paper that motivates the broker integration.

The crate deliberately contains no Buzz, Nostr, channel, membership, or
grant policy; those remain in the Buzz adapter.

---------

Signed-off-by: Jordan Mecom <jm@squareup.com>

Cargo.toml

@@ -2,6 +2,7 @@
 members = [
     "crates/buzz-relay",
     "crates/buzz-core",
+    "crates/ifc-core",
     "crates/buzz-conformance",
     "crates/buzz-push-gateway",
     "crates/buzz-db",