Fix reading mode note widget stability

[martin-forge]

Summary

• Added a shared reading-mode widget observer that schedules re-injection when Obsidian removes or rebuilds the preview widget DOM.
• Refreshed task card and relationships widgets on file-open events to support navigation within the same leaf.
• Refreshed visible reading-mode widgets immediately on metadata changes to eliminate rendering lag.
• Preserved same-file note widgets through transient metadata cache misses while still removing stale widgets for confirmed non-task/non-relationship notes.

Closes #2081.

Testing

• Added unit tests covering same-leaf file reuse, MutationObserver re-injection, immediate metadata refresh, and transient cache misses.
• Ran plugin test suites (npm run lint:ts, npm run build:test, and internal unit tests).