fix(sheet): skip hidden and very-hidden worksheets

[nitishagar]

Closes #9 (sheet-level half).

Summary

anydoc rendered entire hidden / veryHidden worksheets as visible content — indistinguishable from visible sheets in both to_markdown() and to_document(). This adds a pure filter at the sheet frontend: sheets whose state is hidden or veryHidden contribute no heading and no table.

calamine 0.36.1 exposes each sheet's visibility via sheets_metadata() (SheetVisible::{Visible,Hidden,VeryHidden}), which shares one backing Vec<Sheet> with sheet_names() — so the Nth metadata entry describes the Nth name. Positional zip (never name lookup) is used so re-ordering cannot reintroduce the bug.

The change is one file, src/formats/sheet/mod.rs:

• the skip runs before worksheet_range(), so a hidden sheet never counts as "unreadable";
• the multi-sheet heading decision now counts visible sheets, so a single visible sheet among hidden ones keeps (or avoids) its heading correctly;
• an all-hidden workbook degrades to an empty Document (Ok), not a misleading "no sheet could be read" error (hidden is a valid state, not unreadable).

Scope

This addresses the sheet-level half of #9. The row/column visibility half genuinely needs reading xl/worksheets/sheetN.xml <row hidden> / <col hidden> attributes that calamine's xlsx reader does not expose — it depends on the in-house parser and is explicitly out of scope.

ODS hidden-sheet filtering is also out of scope here: .ods routes through odf::parse (a different frontend reading content.xml directly), not sheet::parse, so calamine's SheetVisible is irrelevant to that path. Tracked as a follow-up in src/formats/odf/table.rs.

Verification

• cargo fmt --all --check ✅
• cargo clippy --workspace --all-targets --all-features -- -D warnings ✅
• cargo test --locked ✅ (185 lib + 10 snapshot; existing snapshots byte-for-byte unchanged)
• New tests: hidden_worksheets_do_not_render (visible+hidden+veryHidden fixture → only visible renders), all_hidden_workbook_yields_empty_document (all-hidden → empty Document, not Err/panic), veryHidden covered by the same fixture.
• Branch rebased on current main.

Manual note (from the plan)

On the issue's openpyxl repro (visible row + hidden row + hidden column + hidden sheet), the hidden sheet disappears while the hidden row/column still render — confirming this does not accidentally touch the out-of-scope row/column half.

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