Merge the reviewed Sync design (r28) into NIP-AR and replace the
"later sync specification" note. Sync is a REQ mode that sends the
complete current set at one snapshot, then current heads, and a
resync whenever the relay cannot keep the copy exact. Also defines the
head view, derived identity, home channel lifecycle, and the NIP-11
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Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Signed-off-by: Logan Johnson <loganj@squareup.com>