Read PPTX merge attributes from table cells to preserve merged content

[Floating-Y]

Why

DrawingML stores gridSpan, hMerge, and vMerge on a:tc, but the PPTX parser read them from a:tcPr. Correctly generated slides therefore lost their merge information. The existing fixtures used the same incorrect attribute location, and the empty vertical continuation could pass without reading vMerge.

Follow-up to #1023 and #1042.

What changes

• Read merge attributes when entering a:tc, preserving the existing defaults and boolean parsing. Remove both a:tcPr merge-reading branches so neither XML event form can overwrite cell state.
• Keep horizontal continuations skipped, vertical continuations empty, and the existing table rendering and text handling unchanged.
• Correct the fixture helpers and exercise both <a:tcPr/> and <a:tcPr></a:tcPr>. Nonempty diagnostic markers in horizontal and vertical continuations must disappear, while the expected table shape and values remain.

How it was checked

Local verification during implementation:

• Corrected fixtures against the old parser: 9 passed, 2 failed, exposing both diagnostic markers.
• cargo test -p utopia-ingest --test pptx_tables: 11 passed after the fix.
• cargo test -p utopia-ingest: 159 passed, including related table, single-column, and surrounding-text coverage.
• cargo clippy -p utopia-ingest --all-targets -- -D warnings: passed.
• cargo fmt --all --check: passed.
• Generated a complete two-slide PPTX with python-pptx (40 package parts), inspected the actual cell attributes, and verified both merged-table outputs through the existing render_doc example. This is separate from the minimal parser fixtures; no PowerPoint UI validation was performed.

Before submission, rebased onto dev at 663881d; the intervening upstream changes only touch utopia-reason/src/derive.rs, with no ingest or dependency changes. Reviewed the resulting two-file diff and reran git diff --check. Full workspace Clippy/tests and frontend checks were not run locally; All six CI jobs passed for commit 8fce6f4: CI run.

Before review

• Every commit is signed off (git commit -s).
• cargo fmt --all --check and the ingest-specific tests and Clippy checks pass locally.
• Full workspace Clippy, tests, and build pass in CI.

[WaylandYang]

Thanks @Floating-Y. You are right, and I missed it when I reviewed #1042: the fixtures put the merge attributes where the parser looked, so the test and the parser agreed with each other and not with DrawingML.

Checked with a real file written by python-pptx, one horizontal and one vertical merge. It writes <a:tc gridSpan="2"> and <a:tc hMerge="1">, with an empty <a:tcPr/> after. On dev the header row reads | Summary | | Total |; on this branch | | Summary | Total |, the grid renderer's shape for a spanning header. Landing it.