Document Sequenced* limitations clearly and add more primitives

[sfloess]

Issues

• FileBackedSet throws UnsupportedOperationException for addFirst/removeFirst etc. (expected for append-only design)
• Limited primitive support (suggest LongList, DoubleList etc.)

Actions:

1. Expand README/Javadocs on tradeoffs vs full SequencedCollection contract.
2. Add requested primitive types as per TODO in README.