Pagination: ignore empty spacer <p> and apply its hide mutation in deferred Preview queue

[mettta]

This PR combines the empty-spacer paragraph fix with a minimal deferred mutation pipeline.

Problem:
User-generated HTML may contain empty spacer

nodes. When such a paragraph becomes a page-start candidate near forced breaks, pagination can produce an unjustified blank page.

Solution:

1. Empty spacer paragraph handling:

• Added a dedicated structural predicate for ignorable spacer paragraphs.
• During page-start registration, such paragraphs are skipped as page-start candidates.

2. Deferred mutation pipeline:

• Added a lightweight mutation queue.
• Added mutation commands, including hide-ignorable-spacer-paragraph.
• Pages now registers the spacer hide as a deferred mutation instead of mutating DOM immediately.
• Preview flushes deferred mutations at create-time.

Result:

• The empty spacer <p> case is handled safely.
• DOM writes for this case are now deferred to the Preview stage, reducing immediate layout-impacting mutations during pagination calculations.