Add decimal(n,f) to core types (Tier 2)

Add fixed-point decimal as a core DataJoint type, allowing it to be
recorded in field comments using :type: syntax for reconstruction.
This provides scientists with a standardized type for exact numeric
precision use cases (financial data, coordinates, etc.).

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude

docs/src/design/tables/storage-types-spec.md

@@ -66,6 +66,7 @@ MySQL and PostgreSQL backends. Users should prefer these over native database ty
 | `uint64` | 64-bit unsigned | `BIGINT UNSIGNED` |
 | `float32` | 32-bit float | `FLOAT` |
 | `float64` | 64-bit float | `DOUBLE` |
+| `decimal(n,f)` | Fixed-point | `DECIMAL(n,f)` |
 
 ### String Types
 

src/datajoint/declare.py

@@ -44,6 +44,8 @@
     "varchar": (r"varchar\s*\(\d+\)$", None),
     # Enumeration
     "enum": (r"enum\s*\(.+\)$", None),
+    # Fixed-point decimal
+    "decimal": (r"decimal\s*\(\d+\s*,\s*\d+\)$", None),
 }
 
 # Compile core type patterns