test(ui): guard the interpolated placeholders that the language packs require

[argszero]

Summary

The four key-resolution tests added in #170 check that every key exists, but nothing checked that a call site actually passes the variables the text needs. ui/js/i18n.js leaves the placeholder verbatim when a variable is missing:

s = s.replace(/\{(\w+)\}/g, (m, k) => vars[k] !== undefined ? String(vars[k]) : m);

so T("cnt.calls") renders as {n} 次 / {n} instead of 1,234 次 / 1,234. That defect fails no existing assertion — the key exists, both packs have it, and every count is unchanged — so today only a human eye can catch it.

This adds one more assertion to the same module (src/i18n_pack.rs), plus its negative control:

• every T("key", { … }) call site in ui/js/app.js supplies all placeholders that either pack's text for that key uses (487 call sites, 0 violations today);
• no value bound by a static data-i18n* attribute contains a placeholder — those are written by applyStatic() via innerHTML and have no argument channel at all, so such text would be wrong permanently (305 bound keys, 0 today).

The call-site scanner reuses the module's existing T("literal") recognition rule instead of reimplementing it: an earlier draft with a "smarter" rule found 517 call sites instead of 520, i.e. it counted a different set than every_t_literal_resolves. The tests now share one primitive, and the call-site count is asserted against the existing T_LITERAL_COUNT control.

Related Issue

None. This is a test-only change (CONTRIBUTING sanctions the test commit type and requires #[cfg(test)] tests for new functionality), and the repository has no open issue.

Changes

• 功能/修复说明: no functional change — tests only. No production code, no new dependency, no change to any language pack, ui/ file or behaviour. Adds 2 tests to the existing #[cfg(test)] mod i18n_pack (still compiled only in test builds).
• 涉及配置/数据结构的改动已同步示例文件: N/A — no config or data-structure change.

Tests

• cargo test 全部通过 — 155 passed (was 153)
• cargo fmt --check 通过
• 新增/更新了单元测试（如适用） — 2 added, both with explicit positive controls

The new tests are not merely green on today's tree — they are proven able to fail. Injecting (a) a dropped { n: … } argument at a real call site, (b) a renamed variable ({ count: … }), and (c) a placeholder on a statically bound key each turned the new assertion red with a message naming the offending key. The UI files were restored byte-for-byte afterwards (md5 verified against git show HEAD:…).

Checklist

• 分支命名符合约定（feat/ /fix/ /docs/ ...） — deviation: used test/i18n-placeholder-gate. CONTRIBUTING lists feat/ fix/ docs/ refactor/ but does not list test/, while test is an allowed commit type and nothing but tests changed; the same deviation was taken and disclosed in test(ui): guard the language-pack invariants that were previously unchecked #170.
• Commit message 使用 Conventional Commits 格式 — test(ui): …
• 单一职责，改动最小化 — one assertion class, one file, +495/−18 lines in src/i18n_pack.rs