refactor: simplify diffItem control flow

[ricokahler]

This PR refactors the main diffItem function to improve its control flow and readability. The changes make the logic for handling different data type comparisons more explicit and streamlined.

Key Changes:

• Early Exit for Equality:
  • The function now immediately returns if source === target at the very beginning.
• Dedicated Type Handlers:
  • The logic for determining aType, bType, aIsUndefined, bIsUndefined, dataType, and isContainer has been removed.
  • Instead, diffItem now has explicit checks and calls to dedicated handlers for common type pairs:
    • if (typeof source === 'string' && typeof target === 'string') calls diffString(...).
    • if (Array.isArray(source) && Array.isArray(target)) calls diffArray(...).
    • if (isRecord(source) && isRecord(target)) calls diffObject(...). (Introduced isRecord type guard).
• Simplified Primitive/Fallback Handling:
  • Removed the diffPrimitive function. String diffing is now handled by diffString.
  • The logic for other primitive types or type mismatches is now handled by fallbacks:
    • If target === undefined, an unset patch is pushed.
    • Otherwise (implying a type change or a primitive value change other than string-to-string), a set patch with target is pushed.
• Removed PrimitiveValue Type: The PrimitiveValue type alias is no longer needed as specific type checks are used.

Rationale:

• Improved Readability: The new control flow is more direct and easier to follow. Instead of complex type determination upfront, it checks for specific common scenarios (string-to-string, array-to-array, object-to-object) and then handles the remaining cases (target is undefined, or a general set/replacement).
• Clearer Type Handling: Explicitly calling diffString, diffArray, and diffObject makes the intent clearer for these common structured types.
• Reduced Complexity: Eliminating intermediate type variables (aType, bType, etc.) and the diffPrimitive function simplifies the overall structure of diffItem.
• No Functional Change in Output: These changes are purely a refactor of the control flow and do not alter the diffing logic or the generated patches themselves. The test suite continues to pass, confirming this.

This refactoring makes the core diffing logic within diffItem cleaner and more maintainable.

[ricokahler]

• docs: overhaul README for v6 with API reference and collaborative editing focus #45
• refactor: rename itemA/itemB to source/target #44
• refactor: simplify diffItem control flow #43 👈 (View in Graphite)
• feat: add key-based reordering support for keyed object arrays #41
• refactor: preserve order in patch serialization #42 : 1 other dependent PR (#40 )
• feat!: replace diffItem with diffValue #39
• feat!: remove undefined-to-null conversion warnings and simplify internal APIs #38
• refactor: simplify array unset logic by removing revision lock check #37
• feat!: replace configurable DMP with perf-based heuristics #36
• main

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