fix: preserve parsed picture rotation

[developer0hye]

Summary

• parse a:xfrm rot while reading DrawingML pictures
• normalize the OOXML angle and round it to the nearest whole degree supported by Pic.rot
• add regression coverage for 20933656 -> 349

This compatible backport lets office2pdf preserve the floating picture angle while upstream PR bokuweb#920 is pending.

Validation

• red regression test before implementation: left: 0, right: 349
• targeted regression test passed on Rust 1.81
• cargo test -p docx-rs --lib -- --test-threads=1 (276 passed)
• changed file passed rustfmt +1.81 --edition 2021 --check
• all-target/all-feature Clippy passed after allowing seven pre-existing branch-wide lints
• git diff --check

Existing branch baselines

• cargo fmt --all -- --check reports unrelated pre-existing formatting in fit_text.rs and elements/mod.rs.
• make test reaches 309 passing unit tests, then 20 existing platform-sensitive snapshot mismatches; generated snapshot changes were discarded.
• the unmodified make lint reports seven pre-existing warnings outside this change.

Related: developer0hye/office2pdf#1366
Upstream: bokuweb#920

[developer0hye]

The branch-specific implementation is validated by both build and Clippy jobs, and the new picture-rotation parser test passes locally on Rust 1.81.

The remaining CI failures reproduce on the unchanged fix/parse-tolerance baseline and are outside this PR:

• test: existing platform-sensitive snapshots fail first at read_bom, read_bookmark, read_comment, and read_comment_in_delete_in_insert; this change only touches docx-core/src/reader/pic.rs.
• build-wasm: the current toolchain reports TS5108 (moduleResolution=node10 removed) and TS5011 (rootDir must be explicit) from the existing tsconfig.node.json.

Upstream bokuweb#920 runs the same parser change against current upstream and all four CI jobs pass.