Feedback drops empty-stamped messages; stamp deserialization sorts

[frankmcsherry]

Addresses the timely items from Materialize's report on the differential master-next port.

Changes

• The feedback edge drops a message whose advanced stamp is empty.
  This now includes messages that arrive with empty stamps, which were previously forwarded and could circulate a loop unseen by progress tracking.
• The feedback docs and comment now say that the edge maps capabilities and does not inspect contents.
  Advancing times carried by contents, and discarding those that cannot advance, is the container's job (as differential's ResultsIn does before connect_loop).
  The previous comment claimed that contents supported only by discarded stamp elements were not sent, which the code did not do.
• Deserializing a Stamp sorts its elements, as the bytes need not come from a valid Stamp.
  This adds a T: Ord bound to Stamp's Deserialize, Event's Deserialize, Message's Bytesable, and EventReader's EventIterator.
• The Event::Messages doc no longer calls the stamp an antichain.
• Stamp's doc notes that equality compares multisets: {a} and {a, a} differ.

Not included

• A borrowing accessor for capability times: CapabilitySet derefs to [Capability<T>], so caps.iter().map(|c| c.time()) already borrows.
• A lattice meet over a capability set: timely has no lattice concept, so this belongs in differential.
• A release-mode check of output capability validity.
• The same sorting guard for the Columnar derive on Stamp.

Tests

• progress::stamp::tests::deserialize_sorts
• tests/stamps.rs::zero_capability_stamps_feedback, which fails on the previous behavior.

🤖 Generated with Claude Code

[antiguru]

Looks good, thanks!