[Sskift]

Summary

Adding a key through an out-of-order table proxy updates the document but leaves the proxy's key-to-fragment map unchanged. Deleting or popping that key through the same proxy then raises NonExistentKey. Replacing a newly added table with a scalar can also leave both definitions in the document, producing invalid TOML.

Record the chosen fragment when inserting a new key. Keep the existing placement rules and reuse the fragment index selected during that search.

Validation on Windows / CPython 3.12.10:

• The 14 regression cases produce 12 failures and two passing controls against the original source.
• All 1,238 tests pass with the fix, including the conformance corpus.
• Ruff lint/format and git diff --check pass.
• Mypy reports the same 33 baseline diagnostics after normalizing line numbers.
• A separate Codex agent independently reviewed this commit: 864 operation combinations pass (360 fail on the original source), along with 12 derived mapping operations. It confirmed 48 insertion renderings and eight existing-key operation renderings are unchanged. This is AI review, not human review.

Duplicate Check

• I searched the open PRs and found none that already fixes this issue.
• Related PR Keep out-of-order table indices valid after a table is removed #635 maintains fragment indices after removing an empty table; Fix sibling data loss when updating out-of-order tables #677 changes replacement of existing keys across fragments. This change records the location of a newly added key.

Agent Drafting Metadata

• Agent: OpenAI Codex
• Model: Exact model identifier was not exposed to this worker.
• Notes: Codex assisted investigation, implementation, regression tests, and this description. No human review is claimed.