test(forge): cover YAML frontmatter quoting for special property values

[MarcoPorcellato]

Context

The Obsidian exporter writes page properties as YAML frontmatter. Basic frontmatter is covered, but quoting behavior for YAML-sensitive values is not directly specified.

GFI-59 — test only, estimated 1–3 hours.

Task

Add table-driven tests for the existing frontmatter helpers; keep export behavior unchanged.

Suggested files

• tests/test_forge.py
• Reference: src/logseq_matryca_parser/forge.py

Acceptance criteria

• Plain scalars remain unquoted when safe.
• Values containing a colon, quote, backslash, or newline render as valid deterministic YAML.
• YAML-sensitive property keys are quoted when required.
• Empty properties still produce no frontmatter.

Out of scope

• Changing YAML serialization policy
• Adding a YAML dependency
• Changing the Obsidian export CLI

Verify

uv run pytest tests/test_forge.py -q
make all

No changelog entry is required because this is test-only.