NOW.md prepend format guarantees a conflict between any two concurrent PRs

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Measured cost on 2026-08-19 alone: three manual conflict resolutions on docs/NOW.md (PR #2216 twice against wave merges, PR #2230 once), each identical in shape — two prepended entries, resolution = keep both. The L1 requirement (every PR updates NOW.md) plus prepend-at-top makes the conflict structural: any two PRs in flight collide. Options for an owner decision: (a) directory of entry files docs/now/-.md + a generated NOW.md (no shared hunk, no conflict); (b) .gitattributes 'docs/NOW.md merge=union' — helps local merges but GitHub's web merge does not honor custom merge drivers, so it only halves the pain; (c) accept the tax as a forcing function for serialized landings. No change made — the mechanism is constitutional (L1). Refs #2229.

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Option (a) from this issue — "directory of entry files docs/now/<date>-<slug>.md + a generated NOW.md (no shared hunk, no conflict)" — has landed on master. Closing on that evidence.

The commit

f5be7dc1c6743e6059dee06461fdacdbabb65c3d — "fix(ci): one NOW entry per file, and retire merge=union", PR #2298, merged 2026-08-21T07:21:02Z. Verified an ancestor of master (compare/f5be7dc1c674...master returns behind_by: 0). It did not reference this issue, which is why this did not close automatically.

The shared hunk is gone

docs/NOW.md now opens with:

> **FROZEN ARCHIVE -- do not add entries here.**
...
> New entries go in [`now/`](now/), one file per unit of work:
> `docs/now/<YYYY-MM-DD>-<slug>.md`. Create one with
> `./scripts/tri now add "<title>" --bullet "<what changed>" --closes <N>`.

and docs/now/ exists on master with a README.md plus nine dated entry files, e.g. docs/now/2026-08-20-retire-the-now-md-single-file-bottleneck.md, docs/now/2026-08-21-struct-body-terminator.md. Two PRs in flight now write two different paths, so the first-line collision this issue measured is structurally impossible.

Option (b) was evaluated and explicitly rejected

.gitattributes on master carries the reasoning rather than the rule:

# NOW entries: `merge=union` RETIRED -- there is nothing left to union.

with three recorded reasons, including the one this issue anticipated — "GitHub never applied it. git merge-tree against master with these rules in force reports docs/NOW.md CLEAN for PRs that GitHub simultaneously labels CONFLICTING."

Not closed here: the root NOW.md / docs/NOW.md divergence. That is #2253 and is still open — the same .gitattributes comment points at it.

Triage pass, 2026-08-21. Verified against origin/master @ fead099c213563ca214a85a37956c5cf6f514516 and the GitHub commits API.