34 support upsert into empty documents without sentinel workaround

[nathanjmcdougall]

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[Copilot]

Pull request overview

This PR adds first-class support for mutating “empty” YAML documents (no root data node) so that structure-creating operations (upsert, add, and therefore sync) can bootstrap a root mapping without requiring a sentinel workaround (Issue #34).

Changes:

• Extend Document.add() / Document.upsert() to detect empty documents and bootstrap root mappings when creating new paths.
• Introduce a new native helper _core.serialize_value() (Rust → Python) to serialize Python values to YAML for bootstrapping.
• Add test coverage for mutations and error cases on empty / whitespace-only / comment-only documents, plus a design spec describing the approach.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_document.py	Adds tests for upsert/add/sync behavior on empty documents and verifies expected errors.
src/yamltrip/document.py	Implements empty-document detection and bootstrapping logic for add()/upsert().
src/yamltrip/_core.pyi	Updates native module stubs to expose serialize_value.
src/lib.rs	Exposes serialize_value from the Rust _core module to Python.
src/convert.rs	Implements serialize_value using existing Python→YAML conversion + serde_yaml.
doc/specs/2026-05-22-empty-document-mutations-design.md	Documents the design/behavior for empty-document mutations (Issue #34).

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.