Add Parent-ID header support

[nobbs]

This PR adds stable parent resolution by Confluence page ID via a dedicated Parent-ID header.

Details

• add Parent-ID header support for stable parent resolution by Confluence page ID
• validate that Parent-ID pages belong to the specified space, and warn when Parent headers are ignored
• document the new header and add metadata parsing tests

Testing

• go test ./... (fails in d2 and mermaid image tests; same failures are present on master with differing expected dimensions)

Notes

• Parent-ID is header-only (no CLI flag) and is single-use
• when Parent-ID is set, Parent headers are ignored and logged as a warning
• fixes: Page ID #644
• fixes: FR: user page ID instead of page name for publication #523