feat: Add machine-readable digest of comparison

[Marius Merkle (MariusMerkleQC)]

Motivation

See #27.

Changes

I debated a few different architectural options:

1. First, I added a digest.py file in parallel to the summary. There was quite some duplication, as depending on the input parameters of summary() like top_k_column_changes, slim, etc., different data is put into the summary (and should therefore also be serialized). To avoid this, I then switched to the second option.
2. Here, a new data class SummaryData is populated at initialization of the Summary class. The data can then be used for both
   2.1 Rich-rendered summaries
   2.2 Serialized JSON

The second options avoid code and logic duplication entirely but requires a significant refactor of the summary.py module.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8e2216f) to head (bd9aa41).

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #28    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           10        10            
  Lines          776       921   +145     
==========================================
+ Hits           776       921   +145     

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[Marius Merkle (MariusMerkleQC)]

This uncovered a bug in the summary fixtures, too.

[Marius Merkle (MariusMerkleQC)]

Will remove before merging, I used this as a plan for Claude. It's outdated by now.

[Marius Merkle (MariusMerkleQC)]

The changes below mostly reference the _data instead of the _comparison object.

[Copilot]

Pull request overview

This PR adds a machine-readable JSON “digest” for DataFrameComparison.summary() output by refactoring diffly.summary to compute a structured SummaryData once and using it for both Rich rendering and JSON serialization, plus wiring a new --json CLI flag.

Changes:

• Refactor Summary to compute/store SummaryData and add Summary.to_json() for JSON serialization.
• Add --json to the CLI to output the JSON digest instead of the Rich-formatted summary.
• Extend tests and update Rich-output fixtures to reflect the refactor (notably the Columns table formatting).

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
diffly/summary.py	Introduces SummaryData* dataclasses, computes them in _compute_summary_data, refactors rendering to use them, and adds Summary.to_json().
diffly/cli.py	Adds --json flag and switches output between summary.format() and summary.to_json().
tests/cli/test_cli.py	Parametrizes CLI smoke test to cover both Rich output and --json output.
tests/summary/test_summary.py	Adds parametrized JSON-digest assertions and unit tests for _to_python() conversions.
lexical-sprouting-scroll.md	Adds a design/architecture note describing the refactor and JSON digest approach.
tests/summary/fixtures/many_pk_columns/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/many_pk_columns/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/many_pk_columns/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/many_pk_columns/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/lost_rows_only/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/lost_rows_only/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/lost_rows_only/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/lost_rows_only/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/gained_rows_only/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/gained_rows_only/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/gained_rows_only/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/gained_rows_only/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/equal_non_empty_different_columns/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/equal_non_empty_different_columns/gen/pretty_True_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/equal_non_empty_different_columns/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_True_sample_pk_True.txt	Updates expected Rich output fixture after Columns-table formatting changes.
tests/summary/fixtures/equal_non_empty_different_columns/gen/pretty_False_perfect_True_top_True_slim_False_sample_rows_False_sample_pk_False.txt	Updates expected Rich output fixture after Columns-table formatting changes.

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