docs: split the closed backlog items out to a companion file

Item 73. The backlog kept every item's full Observed/Cause/Approach section
forever, including the sixty-eight that are closed, and had reached 5056 lines:
past the point where it could be read in one pass, and past the point where a
tool could open it at all.

The closed sections move to 2026-08-03-post-0.1.0-usability-closed.md, taking
the backlog to 570 lines. The status table stays where it was and remains the
index of all 80 items, so a closed item keeps its row, its date and its outcome
beside the open ones; only its evidence moved. Nothing was renumbered and
nothing was deleted, which the item required: the numbering is cited from commit
messages, from CLAUDE.md and from the specs, and both files share one sequence,
so item 42 is `## 42.` in whichever file holds it.

The split was done by script and verified by set difference rather than by
reading: every non-blank line of the original appears in one of the two files,
zero missing, and the only lines not in the original are the new file's header.
All 80 numbers resolve, every open item has its section in the backlog, every
closed one in the archive, with no duplicates and no orphans.

Two things the item's own approach did not anticipate. Three cross-references
said "see below" and their targets had just moved, so rows 60 and 75 and the
header's note on item 20's parked branch now say where the entry went. And the
cause was never the fifty done sections, it was that nothing moved a section on
the day its item closed; doing this once buys a few months and then item 73
returns. The rule in "Adding to this document" now requires the move on the
closing commit, and CLAUDE.md tells a future session that grepping the backlog
for a closed item's evidence will find the table row and nothing else.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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