Web: the isZh migration is losing ground — add a one-way ceiling so it can converge

[Lstarsky0]

Problem

The #5337 migration is not converging. Counting files outside web/lib/i18n/ that
branch on locale === "zh", on main:

• 90 days ago: 12
• 60 days ago: 15
• 30 days ago: 27
• today: 31

Over the last 30 days, 10 files gained a branch and 6 lost one. All 6 are
migration work — four came out with the #4934 chrome and homepage move
([locale]/layout, [locale]/page, nav, footer), two with the docs shell
in #5488. The 10 are almost all new docs/* pages, written with the ternary
from the first commit.

Two of those 10 are docs/constitution and docs/runtime-api, which I migrated
in #5517 — written and migrated inside the same month. That round trip is the
whole problem in one line: nothing tells the author of a new page that the
ternary is not the pattern any more, so the copy goes in the page, and then a
later PR moves it out again.

check-locales.mjs cannot see this. It holds dictionaries to parity with the
English reference, so a page whose copy never reaches a dictionary is invisible
to it — exactly the case that needs catching.

Proposal

A one-way ceiling on the set of files allowed to branch, in the shape
scripts/check-persistence-backlog-budget.py already uses here: a committed
document that CI enforces and that can only move in one direction.

web/scripts/check-i18n-ratchet.mjs plus web/scripts/i18n-ratchet-baseline.json
holding today's 31 paths. Two failures:

• a file that branches and is not on the list — a new leak, the message names
  the file and points at lib/i18n/dictionaries/
• a file on the list that no longer branches — a stale entry, the message says
  to delete the line

The second one is what makes it a ratchet rather than a snapshot: migrating a
page means deleting its line, and the ceiling drops by one. It also means the
list is #5337's remaining checklist, maintained by CI instead of by hand.

Detection is the literal comparison, [!=]== "zh". That is deliberately narrow.
A boolean passed down as a prop is not matched directly — admin/admin-client.tsx
takes isZh from its parent — but the site that computes it always is, so a
branch cannot enter the tree without failing somewhere. Matching the bare
identifier instead would be worse: components/nav.tsx mentions isZh in a
comment saying it has none, and lib/public-copy.test.ts asserts on isZh
strings to prove copy is not hardcoded. Both would be false failures on a
required check.

I have the script written and exercised: it passes on the 31-file baseline,
fails on a probe file added to components/ naming that file, and fails when a
migrated path is left on the list telling you which line to delete.

Offer

I'll send this as one PR — script, baseline, a step in the Lint job next to the
existing check-locales.mjs line, and a note in docs/LOCALIZATION.md. Two
questions first:

1. Required check, or run it advisory for a release first? The PR intake gate is
   in dry-run mode right now and this could follow the same path.
2. Scope. Today's scan catches app/api/admin/post/route.ts and lib/page-meta.ts
   alongside the pages. Is Web: finish the #4934 dictionary spine — retire every isZh branch and inline { en, zh } module #5337 meant to cover those, or should the ceiling be
   page copy only?

[github-actions]

Thanks @Lstarsky0 for the report.

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[Hmbown]

The one-way ceiling landed: web/lib/i18n/iszh-ceiling.test.ts fails web CI if files branching on isZh outside the i18n spine exceed 28 (the count on main today), and the constant ratchets down as branches migrate (#5805, 8e4b6822c). Converging the remaining files continues under the ceiling.