refactor(validate): split rendering out of schema_validator.go into renderer/validation subpackage

[jcogilvie]

Context

Follow-up cleanup after #347 (the upstream validate-refactor) merges.

Today, cmd/diff/diffprocessor/schema_validator.go mixes validation logic (scope checks, CRD ensuring, defaulting, calling pkg/validate.SchemaValidate) with ~80 lines of pure formatting helpers — formatValidationErrors, formatMissingSchemaBlock, formatInvalidBlock, formatErrorLine, renderBadValue. The structured-output converter validationFailuresFromResult lives in a sibling file (errors.go).

This is the same business-logic-vs-presentation split we already do for diffs (renderer/diff_renderer.go, renderer/structured_renderer.go). Validation should follow the same pattern.

Proposed shape

cmd/diff/renderer/validation/
  text.go           # Format(*pkgvalidate.ValidationResult) string
  structured.go     # ToFailures(*pkgvalidate.ValidationResult) []dt.ResourceValidationFailure
  validation_test.go

• schema_validator.go calls validation.Format to build SchemaValidationError.Message.
• errors.go calls validation.ToFailures from NewOutputError.
• Wire types (ResourceValidationFailure, FieldValidationError) stay in renderer/types/types.go — they're already there.

Constraint

The eager-vs-lazy timing of SchemaValidationError.Message stays unchanged. Go's error interface needs .Error() string synchronously, so the text rendering can't become as lazy as the diff renderer's. The location of the formatters moves; the timing does not. This is the one place the analog with the diff renderer diverges.

Scope

• Pure code-move plus import updates
• No behavior change
• Estimated ~+200 lines of churn (new package, moved functions, moved tests, updated imports)
• All mechanical; reviewer cost should be low

Why now (after #347)

#347 is already substantial — upstream API consumption, structured output exposure, integration test rework, dead-state sweep. Doing the split inside that PR would dilute its narrative. After #347 merges this becomes a focused refactor.