fix(PC0037): suppress same-field assignment in own validate trigger

[Arthurvdv]

Closes #357

Problem

PC0037 (UseValidateForFieldAssignment) raised a false positive when a field is assigned to the current record (Rec/self) inside that field's own OnValidate / OnBeforeValidate / OnAfterValidate trigger. That by-design value transformation (default values, rounding a unit price) cannot use Validate() without recursion, so it should not be flagged.

Change

Adds a narrow suppression guard in UseValidateForFieldAssignment:

• Fires only when the assignment is inside a validate trigger, targets the same field the trigger owns, and the instance is the current record.
• "Current record" detection (IsCurrentRecordInstance): a this/self reference, detected via instance.Kind == EnumProvider.OperationKind.ThisReference (guarded != default), or an instance symbol named Rec (covers explicit Rec. and a page's implicit-with bare reference). Rec/xRec are reserved AL keywords sharing one record type, so the name is the only public discriminator from the xRec before-image.
• Owner field resolution (ResolveTriggerOwnerField): from the trigger's containing symbol — IFieldSymbol (table field), IControlSymbol.RelatedFieldSymbol (page control), or, for modify(...) before/after validate in extensions, the change-modify symbol's internal Target (read via reflection, guarded by owner.Kind == SymbolKind.Change, resolved recursively).
• Covers tables, table extensions, pages and page extensions.

Still fires (proves narrowness)

• Different field on Rec inside another field's OnValidate.
• xRec."Same Field" and other same-table record variables.
• Another record variable's same-named field.

netstandard2.1 / SDK-version compatibility

• this/self is detected via the OperationKind.ThisReference enum through EnumProvider, not the IInstanceReferenceOperation type. That type is absent from the netstandard2.1 compile floor (AL 12.0.13), and referencing it would force an #if !NETSTANDARD2_1 guard that silently drops this. suppression on the netstandard2.1 binary serving AL 14.0-15.2. The enum resolves to default on SDKs without the member (where no this code can exist), so the != default guard makes it a no-op there and exact elsewhere — no #if, works on every TFM.
• Adds EnumProvider.OperationKind.ThisReference and EnumProvider.SymbolKind.Change (both sorted into their alphabetical slots).
• Built clean on netstandard2.1 and net10.0.

Tests

src/ALCops.PlatformCop.Test — all 468 pass (PC0037: 20/20).

• NoDiagnostic (new): TableFieldOnValidateSameField, TableExtensionFieldOnBeforeValidateSameField, TableExtensionFieldOnAfterValidateSameField, PageControlOnValidateSameField, PageExtensionControlOnBeforeValidateSameField, PageExtensionControlOnAfterValidateSameField, OnValidateSameFieldThisReference, PageControlOnValidateSameFieldBareReference.
• HasDiagnostic (new): OnValidateDifferentFieldOnRec, OnValidateXRecSameField, OnValidateOtherRecordSameField.
• Runtime guards: the four extension fixtures are skipped below runtime 13.0 (extension target declared in the same module) and OnValidateSameFieldThisReference below 14.0 (the this keyword), via SkipTestIfVersionIsTooLow, matching AC0032 / DuplicateODataEntityName.

Docs

• pc0037-use-validate-for-field-assignment.instructions.md updated (design decisions, architecture, known issues).
• analyzer-development.instructions.md and netstandard21-compatibility.instructions.md gained reusable guidance on detecting this/self via OperationKind.ThisReference at the operation level.

Note: base is release/v1.0.0 per request (deviates from the usual main target).