i18n(terms): protect two key forms, four case variants, and a singular

[bloxster]

Why

Three gaps found while auditing translation/protected-terms.json against the categorized list in the frontend repo.

1. Key and address compounds are only half protected. Unified Address and Viewing Key are enforced; the other expanded forms are not, so a translation can localize them and no check complains.

2. The check is case-sensitive, so a term protected in one casing is unprotected in the other. The list carries Free2Z, Zcash.Me, MetaMask Snap and Zingo, while the variants that actually appear in the wiki went unenforced. Same trap as Zodl vs ZODL, still open.

3. ZK-SNARKs is enforced but its singular is not.

What is added — all seven cost nothing

term	English pages	translations missing it
Full Viewing Key	5	0
Incoming Viewing Key	2	0
Zcash.me	1	0
Zingo!	20	0
ZGo	27	0
Zgo	9	0
ZK-SNARK	1	0

Every existing translation already keeps them verbatim, so this locks in current behaviour and prevents a future regression.

Verified: the whole-tree audit reports 72 violations both before and after, and none names a newly added term. (Those 72 are pre-existing drift from #1948 and #1957, cleared by the pending backfill/re-sync PR.)

What is not here — and why it should not be added later either

An earlier draft of this PR listed Unified Addresses as merely expensive: 94 violations, 113 after the pending re-sync. Measuring what those violations actually are changed the conclusion. Protecting an English plural is wrong in principle, not just costly.

Of the 113, 70% already carry the protected singular Unified Address. They did not lose the concept — they declined to pluralize an English loanword:

• English "for inspecting Unified Addresses" → Italian "per ispezionare gli Unified Address". That is correct Italian; Italian does not add -s to English loanwords.
• Japanese writes Unified Addressを調べる, because Japanese does not inflect plurals at all. 23 of the 113 sit in ja/ko/zh, none of which do.

Enforcing the plural would push a grammatical error into Italian, French and Spanish, and a meaningless string into three more, to satisfy a check the singular already satisfies conceptually.

The principle: protect singulars, not plurals. Languages differ in whether and how they pluralize borrowed nouns, so a protected plural imposes English grammar on all 18 locales.

This is not hypothetical. ZK-SNARKs is enforced today, and on all four English pages using it, ja/ko/zh/it carry the English plural verbatim — they had no choice. A follow-up will teach the checker that a plural is satisfied by its protected singular and retire the plural entries; this PR adds ZK-SNARK so that follow-up has something to fall back to.

Still held back

Priced against both main and the pending re-sync tree. These create violations that no queued sync repairs — the terms list is not part of staleness detection, so the debt would be invisible to the dashboard and unreachable by the sync agent:

TEX Address 39 · Dev Fund 25 (29 after re-sync) · Diversified Address 14 · Spending Key 7 · Metamask Snap 2 · zk-SNARK 2 · Free2z 1

Skipped entirely: Unified Viewing Key and Outgoing Viewing Key are in the categorized list but appear on no English page, so there is nothing to protect yet.