Add Excel .xlsx / ODS spreadsheet parser

[copyleftdev]

Implements the Excel / OpenDocument plugin — issue #21.

What

The first worksheet of a workbook (.xlsx / .xls / .xlsb / .ods) becomes a
RecordSet: the first row is the header (column names), each subsequent row a
record, and every cell maps to the closed Value set — so all detectors apply
with no special-casing. A date/time cell keeps its Excel serial number
(numeric, deterministic); blank and error cells are Null (honest absence).

Reading is delegated to calamine (pure Rust, reads all four formats).

Routing

• Detected by the ZIP magic (PK\x03\x04) plus an xl/ part (xlsx/xlsb) or the
  OpenDocument-spreadsheet mimetype (ods) — so a .docx / .jar / plain zip is
  not claimed. The OLE2-based .xls is covered by extension.
• Confidence MAGIC; extensions .xlsx / .xls / .xlsb / .ods.

Feature gating

Behind the default-on xlsx feature (binary format), so the text-only
--no-default-features build stays lean. Both builds verified.

Testing

The roundtrip test writes a tiny .xlsx in-memory with rust_xlsxwriter
(dev-dependency — no committed binary fixture) and parses it back;
data_to_value and header_name are unit-tested across every cell kind
(int/float/NaN/string/bool/date/empty/error); non-workbook input is a clean
AxError::Parse.

Gates

• fmt / clippy -D warnings / full workspace tests green (default and
  --no-default-features).
• Mutation gate: 0 surviving mutants on xlsx.rs.

Closes #21

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Added support for parsing spreadsheet files in XLSX, XLS, XLSB, and ODS formats
  • Automatic spreadsheet detection now enabled by default
  • Spreadsheet content converted to structured columns and records with header extraction and value mapping

[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

This PR adds optional spreadsheet parsing to ax-normalize using the calamine crate. It introduces a new XlsxParser that detects and parses Excel/ODS files by reading the first worksheet into columns, with header inference and cell-to-Value conversion rules including NaN nullification and date serial preservation.

Changes

XLSX Parser Feature

Layer / File(s)	Summary
Dependency and feature configuration crates/ax-normalize/Cargo.toml	Adds calamine optional dependency and rust_xlsxwriter dev-dependency; introduces xlsx feature flag included in default alongside existing format parsers.
Parser module registration crates/ax-normalize/src/parsers/mod.rs	Conditionally declares xlsx submodule, re-exports XlsxParser, and registers it in default_registry() when feature is enabled.
XlsxParser implementation crates/ax-normalize/src/parsers/xlsx.rs	Implements FormatParser for XLSX/XLS/XLSB/ODS: sniffs via ZIP+spreadsheet markers, parses first sheet with string headers (trimmed) or fallback col{index} naming, maps cells to Value (non-finite floats and blanks→Null, dates→Float serial numbers).
Test coverage crates/ax-normalize/src/parsers/xlsx.rs, crates/ax-normalize/src/parser.rs	Test utilities for in-memory spreadsheet generation; validates worksheet parsing, header rules, value mappings, sniff detection, extension matching, error handling, and registry resolution.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• copyleftdev/anomalyx#29: Both PRs extend the format parser registry in ax-normalize by adding parser IDs (xlsx in this PR; toml/ini in #29) to the module registry and the default_registry_lists_all_formats test.

Poem

🐰 A spreadsheet hops through calamine's gate,
First rows become headers, cells translate,
To Values they flow—no NaN too great,
Dates as floats, tests validate the fate!
One parser to rule them: xlsx's state. ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately and concisely summarizes the main change: adding an Excel/ODS spreadsheet parser to the codebase.
Linked Issues check	✅ Passed	All acceptance criteria from issue #21 are met: XlsxParser implements FormatParser trait, is registered in default_registry, includes roundtrip/sniff/error tests, passes cargo fmt/clippy, and has 0 surviving mutants.
Out of Scope Changes check	✅ Passed	All changes are scoped to issue #21 requirements: new parser implementation, feature gating via Cargo.toml, and supporting test updates. No unrelated modifications detected.
Docstring Coverage	✅ Passed	Docstring coverage is 85.00% which is sufficient. The required threshold is 80.00%.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/xlsx

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