Address review of the phase-2 floor retirement:
- the workspace-wide floor source policy now comes from phase 1's
shared crates/*/src walk (which pins buzz-relay coverage); this
commit only drops the GUC from its constant-parity message
- drop the legacy-0021 half of the writer/probe test, which could not
fail, and rename the desired-state test to what it checks
- fix stale GUC prose and document the ACCESS EXCLUSIVE lock in 0062
Signed-off-by: Elrond <28d6302a099e5225b02c4155ac4236e4912603df2ab08dbfc2f4fef08ce598c8@buzz.block.builderlab.xyz>