docs: define the term as 'schema dimension' (shortened to 'dimension') (#239)

Promote 'schema dimension' to the defined term in its two definitional homes
(Entity Integrity section Schema dimensions; Read Diagrams section Dimensions and
Underlined Names), then shorten to 'dimension' in-context. Canon already used
'Schema dimensions' as the section heading and 'lineage through schema
dimensions' in prose — this makes the defined term consistent and disambiguates
from array and data-warehouse 'dimension'. Headings/anchors unchanged.

Note: read-diagrams cell source normalized to a single string by the notebook
editor; llms-full.txt regenerates from source.

Author: dimitri-yatsenko

src/explanation/entity-integrity.md

@@ -213,7 +213,8 @@ in a single statement.
 
 ## Schema dimensions
 
-A **dimension** is an independent axis of variation in the data. The rule:
+A **schema dimension** — often shortened to *dimension* — is an independent axis of
+variation in the data. The rule:
 
 > **Any table that introduces a new primary-key attribute introduces a new
 > dimension.**

src/how-to/read-diagrams.ipynb

@@ -389,20 +389,7 @@
    "cell_type": "markdown",
    "id": "cell-dim-md",
    "metadata": {},
-   "source": [
-    "## Dimensions and Underlined Names\n",
-    "\n",
-    "A **dimension** is a new entity type introduced by a table that defines new primary key attributes. Each underlined table introduces exactly **one** dimension—even if it has multiple new PK attributes, together they identify one new entity type.\n",
-    "\n",
-    "| Visual | Meaning |\n",
-    "|--------|--------|\n",
-    "| **Underlined** | Introduces a new dimension (new entity type) |\n",
-    "| Not underlined | Exists in the space defined by dimensions from referenced tables |\n",
-    "\n",
-    "**Key rules:**\n",
-    "- Computed tables **never** introduce dimensions (always non-underlined)\n",
-    "- Part tables **can** introduce dimensions (may be underlined)"
-   ]
+   "source": "## Dimensions and Underlined Names\n\nA **schema dimension** (often shortened to *dimension*) is a new entity type introduced by a table that defines new primary key attributes. Each underlined table introduces exactly **one** dimension—even if it has multiple new PK attributes, together they identify one new entity type.\n\n| Visual | Meaning |\n|--------|--------|\n| **Underlined** | Introduces a new dimension (new entity type) |\n| Not underlined | Exists in the space defined by dimensions from referenced tables |\n\n**Key rules:**\n- Computed tables **never** introduce dimensions (always non-underlined)\n- Part tables **can** introduce dimensions (may be underlined)"
   },
   {
    "cell_type": "code",
@@ -3473,4 +3460,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 5
-}
+}