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/URT-v5.1-Reference.md

@@ -6,7 +6,7 @@ The Universal Review Taxonomy (URT) is a classification system for customer feed
 
 ### Key Characteristics
 
-- **Three Profiles**: Core, Standard, Full (increasing detail)
+- **Four Profiles**: Lite, Core, Standard, Full (increasing detail)
 - **Seven Domains**: Covering all aspects of customer experience
 - **Tier-3 Canonical Codes**: Format `X#.##` (e.g., J1.02, P2.15)
 - **Dimensional Annotation**: Valence, intensity, specificity, and more
@@ -129,8 +129,10 @@ USN is a compact string encoding for URT annotations.
 ### Grammar
 
 ```
-Standard: URT:S:{codes}:{V}{I}:{S}{A}{T}.{E}.{CR}
-Full:     URT:F:{codes}:{V}{I}:{S}{A}{T}.{E}.{CR}:{causal}
+Lite:     URT:L:{domain}:{V}{I}
+Core:     URT:C:{category}:{V}{I}
+Standard: URT:S:{subcode}[+{sec}]:{V}{I}:{S}{A}{T}.{E}.{CR}
+Full:     URT:F:{subcode}[+{sec}]:{V}{I}:{S}{A}{T}.{E}.{CR}[:{causal}]
 ```
 
 ### Encoding Rules
@@ -138,6 +140,8 @@ Full:     URT:F:{codes}:{V}{I}:{S}{A}{T}.{E}.{CR}:{causal}
 **Valence**:
 - `+` for V+
 - `-` for V-
+- `0` for V0
+- `±` for V±
 
 **Intensity**:
 - `1` for I1