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