Preserve explicit zero page margins and expose presence

[developer0hye]

Explicitly setting all six print margins to zero currently writes Excel defaults instead. This change distinguishes unset margins from supplied values, so zero survives a write/read cycle and defaults apply only to unset fields.

• Add PageMargins::has_left, has_right, has_top, has_bottom, has_header, and has_footer using the existing internal presence state. Existing numeric accessors retain their behavior.
• Select writer defaults independently for each unset margin; partially configured objects now default their unset fields rather than writing numeric zero for them.
• Cover explicit all-zero margins, completely unset margins, mixed zero/nonzero/unset margins, and a worksheet without a pageMargins element. Tests use synthetic workbooks generated in memory.
• Document the presence API and writer behavior.

The base-compatible zero-margin regression fails on master 35bbffa4 (all six values reopen as defaults) and passes with this change. The default-only control passes on both.

Validation on macOS:

• cargo test --locked: 344 passed, 23 existing ignored tests.
• cargo clippy --locked -- -D warnings: passed.
• cargo clippy --locked --test page_margin_presence -- -D warnings: passed.
• cargo +nightly-2026-09-11 fmt --all --check: passed.
• git diff --check: passed.

The local lockfile is untracked, consistent with this repository. No dependency changes or external fixtures.

Related: #372. Downstream: developer0hye/office2pdf#1812.

[MathNya]

@developer0hye
Thank you for the PR.
Since there were no issues with the changes, I will merge them.