docs(ui): describe the recent-models store the code actually keeps

[argszero]

Summary

ui/README.md's 「市场「最近使用」约定」(v1.20) still described the pre-#255 shape of the
recent-models store: helpers named getRecentIds() / saveRecentIds(), and a stored array of
model ids. #255 (C2138/C2139) renamed those helpers to getRecentKeys() / saveRecentKeys()
and made the stored value the model's identity string (provider/model) — an array index is
not an identity, because it is persisted and then resolved again in a different array
(D.MARKET for a guest session) or in a different catalog order.

Neither dead name exists anywhere in the repository outside that section (two sites, both
ui/README.md), while the same file documents the new contract 220 lines below
(「模型身份：provider/model，不是数组下标（C2138）」) — so the two sections of one document
contradicted each other, and the stale one names two functions a reader cannot find.

Related Issue

None — found while scanning this repository; no issue was filed for it.

Changes

• ui/README.md: the section now names the helpers and the stored value the code actually uses
  (getRecentKeys / saveRecentKeys / identity strings), states that legacy numeric entries
  are dropped, and adds a one-line C2138（#255）更正 provenance note in the style this file
  already uses for corrected sections, pointing at the existing identity section.
• No configuration or data-structure change, so config/config.example.toml needs no update.
• No ui/js/* or ui/index.html change, so no ?v= cache-bust token is bumped.

Tests

• cargo test — 413 passed, 0 failed (same as main)
• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — clean
• No unit test added: the change is Markdown prose, and this repository's standing decision for
  documentation claims is to correct the text rather than to add a gate
  (precedents: docs(perf_gate): drop the stale file count from the gate header #309, docs(ui): stop restating the i18n counters and correct two stale gate claims #310, fix(admin): state the refusal a deleted model now meets, not the old zero bill #312).

The claim itself, measured on the branch tree:

$ git grep -nE "getRecentIds|saveRecentIds" -- .                    # zero hits
$ git grep -c "getRecentKeys\|saveRecentKeys" -- ui/js/app.js       # 3 + 3, the names now cited
$ git grep -n "atp-recent-models" -- ui/js/app.js                   # the storage key, as cited

Checklist

• Branch name follows the convention (docs/…)
• Commit message uses Conventional Commits (docs(ui): …)
• Single responsibility, minimal change (1 file, +4/−3)

[argszero]

Self-review (committer, allow_self_merge): the diff is the three stale lines plus one provenance
note, and the claim it makes is measurable on the branch tree — git grep -nE "getRecentIds|saveRecentIds" returns zero hits after the change and returned two before, both in the
section being corrected. Every identifier the section now cites resolves in ui/js/app.js
(getRecentKeys / saveRecentKeys / markRecentUsed / renderRecent), and the cross-reference it
points at (模型身份：provider/model，不是数组下标（C2138）) is in the same file. CI: both jobs pass
(test / fmt / clippy, msrv). No code, no i18n key, no cache-bust token.

LGTM — merging.