Tier 1: error model, validation window & native [CsvKey] dictionary (v2.4.0)

[zitgatech]

Summary

Tier 1 of the roadmap: a consistent error model, a validation report window, and native
Dictionary columns. Bumps the package to 2.4.0. Editor-only; no runtime cost.

1. Collect-all error model

• New public CsvReaderException + CsvCellError carry (row, column, value, reason).
• Per-cell data errors are collected across the whole file and thrown together instead of
  aborting on the first bad value — fix a whole CSV in one pass. Invalid enum, non-numeric
  number, bad bool and duplicate [CsvKey] are all data errors.
• Structural problems (empty CSV, missing column, unknown type, unconfigured assemblies)
  still fail fast. Nothing is written for a file that has data errors.

2. Validation Report window

• Tools → Csv-Reader → Validation Report lists every data error from the last import batch,
  grouped by file, with an Open CSV shortcut. Focused automatically after a failing import.
• A bad file no longer blocks the rest of the import batch — every file's errors surface at once.

3. Native Dictionary<K,V> via [CsvKey]

• Mark the row type's key field with [CsvKey] and make the target field a Dictionary<K,V>;
  the reader builds it directly — no convert-method boilerplate.
• Duplicate / null keys are reported as data errors. The [CsvKey] type must equal the key type;
  exactly one per row type.
• Scope is one CSV → one dictionary; the holder must serialize dictionaries (Odin
  SerializedScriptableObject). Merged-folder + Dictionary fails fast with a clear message.

⚠ Behavior change (see CHANGELOG)

• A bad cell used to throw FormatException/KeyNotFoundException on the first offender.
  It now throws a single CsvReaderException aggregating every bad cell. Code that caught the
  old specific types around a cell should catch CsvReaderException (exposes CellErrors).
  Structural exceptions are unchanged.

Testing

EditMode tests: 41/41 passing (35 previous + 6 new), verified via the Unity Test Runner.
New fixtures: ReaderErrorCollectionTests, ReaderDictionaryTests.

Commits

1. Collect-all error model, validation window & native [CsvKey] dictionary (core)
2. EditMode tests for error collection and dictionary build
3. Docs + bump to 2.4.0

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