docs(upstream): record the v0.52.1 release

[sunerpy]

Summary

This records the v0.52.1 release in docs/upstream-sync/UPSTREAM.md, as #290 did for v0.52.0. The Current alignment block gains a "Shipped in v0.52.1" bullet for #291 and #292. A new dated release entry covers:

• release workflow run 36879201833, which passed all twelve jobs on its second attempt after a transient Actions-cache 503 broke Windows Clippy on the release commit;
• the published digest;
• the black-box acceptance against the official v0.52.0.

The diff only adds lines; no earlier entry changes.

Verification

make tools-check fmt-check docs-check passes. UPSTREAM.md is in .oxfmtignore. The acceptance results quoted come from /tmp/verify-0.52.1.sh run against the downloaded release:

• the checksums and attestations of all six archives;
• codegraph 0.52.1, built by rustc 1.98.0;
• each of fix: address the retroactive review findings on the v1.6.1 port #291's four fixes, compared with v0.52.0;
• the upgrade of a v0.52.0 index: outdated, then current after sync.

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[chatgpt-codex-connector]

Record the implementation-to-release provenance

When this append-only ledger is later used to audit v0.52.1, the entry begins with release PR #293 and never records the immutable PR-head and merge SHAs for implementation PRs #291 and #292. Consequently, it does not preserve the required implementation-head → merge → tag chain even though it claims those changes shipped; add the exact implementation provenance before the tag/workflow evidence.

AGENTS.md reference: AGENTS.md:L186-L187

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
- Coverage   95.31%   95.31%   -0.01%     
==========================================
  Files         161      161              
  Lines       99130    99130              
==========================================
- Hits        94487    94484       -3     
- Misses       4643     4646       +3     

see 2 files with indirect coverage changes

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