fix: ensure root KV are properly inserted at the top of the document

[DecimalTurn]

Fixes #170

[Copilot]

Pull request overview

Fixes incorrect placement of newly-added root-level key/value pairs so they’re inserted into TOML’s implicit root table (before the first [table] / [[array-of-tables]] header), addressing issue #170.

Changes:

• Clamp insertion index for new root-level KeyValue nodes to the end of the implicit root table scope (before the first table/AoT header).
• Adjust writer line-offset logic when prepending into a non-empty document to correctly shift existing content down.
• Add regression tests for root key/value placement relative to tables and AoT sections.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/writer.ts	Adds special-case handling for inserting at the start of a non-empty document so existing nodes are shifted correctly.
src/patch.ts	Ensures root-level key/values are inserted before the first explicit table/AoT section rather than appended at the document end.
src/__tests__/patch.test.ts	Adds coverage for root key/value placement before sections/AoTs (and includes one currently skipped inline-table scenario).

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