Centralize Plutus target metadata and split script purpose from type

[satran004]

Description

This PR keeps JuLC’s compiler output V3-only, but removes scattered hardcoded V3 metadata and separates script language type from validator purpose.

Changes:

• Adds PlutusTarget.CURRENT as the single source of truth for compiler target metadata:

  • language version: v3
  • text envelope type: PlutusScriptV3
  • UPLC program version: 1.1.0

• Replaces direct Program.plutusV3(...) compiler output construction with PlutusTarget.CURRENT.program(...).

• Removes output-formatting responsibility from JavaSourceIntrospector.

• Adds ScriptPurposeMetadata for purpose formatting.

• Changes generated validator JSON from combined type strings like:

  • PlutusScriptV3-Minting

  to:

  • type: PlutusScriptV3
  • purpose: minting

• Preserves backward compatibility when reading old JSON files with legacy combined type strings.

• Updates Gradle plugin, annotation processor, CLI blueprint conversion, and blueprint generation to use the centralized target metadata.

• Updates architecture docs to clarify:

  • JuLC compiler output targets V3.
  • VM evaluation can still handle V1/V2/V3 external scripts.