Fix invalid dates in feed ordering

[cardmagic]

A malformed pubDate could make latest raise or hide a valid updated or published timestamp. Use the first valid Time across those fields consistently in latest, items_since, and merge while preserving the original field values.

Sort dated entries newest first, retain source order for ties, and place undated entries after even pre-1970 dates. Preserve merge's identity rules and its placement of unidentified entries at the end. Document the behavior and cover fallback dates, nanosecond precision, stable ordering, deduplication, and unchanged inputs.

Validation: reproduced the mixed String/Time comparison error with a failing regression before the fix. The full suite passes on Ruby 3.4.8 (112 tests, 233 assertions; 3 existing network-test omissions). RuboCop, RBS validation, and Steep pass.

GitHub CI passes on Ruby 3.2, 3.3, 3.4, and 4.0, and Greptile reports 5/5 on e26a025. Hound's style comments were resolved because they conflict with .rubocop.yml: the repository requires double quotes and excludes tests from line-length and method-length checks.

Closes #58.

[greptile-apps]

Greptile Summary

This PR makes feed date handling consistently select the first valid Time from pubDate, updated, and published.

• Sorts dated entries newest-first with nanosecond precision and stable tie ordering.
• Places undated entries after all dated entries, including pre-1970 dates.
• Preserves merge identity, deduplication, and unidentified-entry placement behavior.
• Adds documentation and regression coverage for malformed dates, fallbacks, ordering, deduplication, and input immutability.

Confidence Score: 5/5

The PR appears safe to merge, with the changed date-selection and ordering behavior covered by focused regression tests.

No actionable correctness, security, or repository-rule violations remain; the implementation preserves existing identity semantics while preventing malformed date fields from masking valid fallbacks.

Important Files Changed

Filename	Overview
lib/simple-rss.rb	Centralizes first-valid-date selection and applies stable, precise ordering consistently to latest, filtering, and merge.
test/base/date_ordering_test.rb	Adds comprehensive regression coverage for fallback dates, precision, stable ordering, merge deduplication, and unchanged inputs.
test/base/enumerable_test.rb	Verifies malformed date values remain accessible while latest ordering succeeds.
test/data/mixed_dates.xml	Supplies representative valid, malformed, blank, missing, historical, fallback, and fractional timestamps.
README.md	Documents the date precedence, ordering stability, filtering, and merge semantics introduced by the change.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Feed entry] --> B{Valid pubDate?}
  B -- Yes --> E[Use pubDate]
  B -- No --> C{Valid updated?}
  C -- Yes --> F[Use updated]
  C -- No --> D{Valid published?}
  D -- Yes --> G[Use published]
  D -- No --> H[Treat as undated]
  E --> I[Stable newest-first ordering]
  F --> I
  G --> I
  H --> J[Place after dated entries]

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