Align artifacts with canonical URT v5.1 specification

Fixes inconsistencies discovered during audit against urt-taxonomy/:

- urt_profile ENUM: Add 'lite' and 'core' profiles (was missing)
- USN format: Use canonical regex from spec (was non-compliant)
- USN valence encoding: Add V0 (0) and V± (±) support
- USN grammar: Add Lite (URT:L:) and Core (URT:C:) formats
- Dimension codes: Fix temporal (TC/TR/TH/TF), evidence (ES/EI/EC),
  comparative (CR-N/CR-B/CR-W/CR-S) in decisions doc
- LLM contract: Full USN regex validation pattern

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.artifacts/LLM-Classification-Contract-v1.md

@@ -243,8 +243,8 @@ Return valid JSON matching the schema exactly. No markdown, no explanations.
           },
           "usn": {
             "type": "string",
-            "pattern": "^URT:S:[OPJEAVR][1-4]\\.[0-9]{2}",
-            "description": "URT String Notation for audit"
+            "pattern": "^URT:S:[OPJEAVR][1-4]\\.[0-9]{2}(\\+[OPJEAVR][1-4]\\.[0-9]{2}){0,2}:[+\\-0±][123]:[1-3][1-3]T[CRHF]\\.E[SIC]\\.[NBWS]$",
+            "description": "URT String Notation for audit (Standard profile)"
           }
         }
       }