feat(pptx): add opt-in hidden slide inclusion

[acpatelmd]

File submission policy

• No sample documents, attachments, or private fixtures are submitted.

Summary

Add an opt-in path for exporting hidden PPTX slides while preserving the existing default that omits them.

• Add ConvertOptions.include_hidden_slides, defaulting to false.
• Add the CLI flag --include-hidden-slides and pass it through to the PPTX parser.
• Include both show="0" and show="false" slides in original presentation order when enabled.
• Keep --slides ranges based on original one-based source positions.
• Update the generated TypeScript binding, regression tests, and README usage.

Related issue

Related: #1755

Testing

These are focused local checks; the full workspace suite was not run locally.

• cargo test --locked -p office2pdf config::tests::test_convert_options
• cargo test --locked -p office2pdf parser::pptx::tests::slide_feature_tests::test_
• cargo test --locked -p office2pdf-cli test_cli_include_hidden_slides
• cargo test --locked -p office2pdf --features typescript config::ts_tests
• cargo test --locked -p office2pdf --features typescript ts_integration_tests
• cargo test --locked -p office2pdf-cli --features server test_parse_query_string
• cargo run --locked -p office2pdf-cli -- --help
• cargo fmt --all -- --check
• git diff --check

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: No slide drawing or layout code changed. Existing/default invocations render the same pages; the opt-in path intentionally adds previously omitted hidden pages and preserves their source order. Synthetic parser tests cover that export-policy change without submitting documents or visual fixtures.

Checklist

• Commits include a Signed-off-by line
• PR scope contains one root cause
• No private documents, fixtures, paths, or company names are included