docs(perf_gate): drop the stale file count from the gate header

[argszero]

Summary

src/perf_gate.rs's module header described its first positive control as asserting that the
scan "found 4 files". The roster it describes holds five entries, and the assert that
pins the roster reads five — the same file already says so one screen further down:

const FILES: &[(&str, &str)] = &[ /* wallet.rs, ops.rs, admin.rs, org.rs, sharing.rs */ ];

assert_eq!(FILES.len(), 5, "应扫描 5 个路由文件");

So the header advertised a positive control that no longer exists in the shape it describes.

The numbers agreed when 93b2ebd (#234) wrote the module; 199566c (#243) added sharing.rs
to FILES and updated only the assert, leaving the header behind.

Changes

• src/perf_gate.rs: drop the count from that header clause and name the mechanism
  (FILES.len() compared against the expected value) instead of restating a number.

Refreshing 4 → 5 was the other option and is deliberately not taken: the count is already
owned by the assert, which fails loudly when the roster changes. A header restating it is a
second carrier of one fact that can drift again unnoticed — which is exactly what happened here.
This follows the repo's existing treatment of stale counts in gate comments (see #180,
docs(i18n): drop the stale per-update count log from the gate comment).

Comment-only; no behaviour, no test, no roster change.

Sibling gates re-checked in the same family

• src/table_gate.rs — header says 7 tables / 7 empty states / 7 failure states; the roster holds
  7 and its asserts derive from TABLES.len(). Consistent, left alone.
• src/body_limit_gate.rs — header says "≥ 20 files"; the assert is files.len() >= 20.
  Consistent, left alone. (Its "今日 0 处" note is explicitly scoped as a known limitation.)

Related Issue

None — no open issue covers this (the repository currently has no open issues).

Changes

• 功能/修复说明 — the stale claim in the gate header is removed; nothing else touched
• 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 touched

Tests

• cargo test 全部通过 — 409 passed; 0 failed (Finished, 18.88s)
• cargo fmt --check 通过 — exit code 0
• 新增/更新了单元测试（如适用） — n/a, comment-only change; the gate's own positive
  controls (FILES.len() == 5, per-file closed-range counts) are unchanged and still green

Also ran cargo clippy --all-targets -- -D warnings locally: exit code 0.

Checklist

• 分支命名符合约定（feat/ /fix/ /docs/ ...） — fix/perf-gate-file-count-claim
• Commit message 使用 Conventional Commits 格式 — docs(perf_gate): …
• 单一职责，改动最小化 — 1 file changed, 3 insertions(+), 3 deletions(-)