feat: Add dqlens diff — compare two profiles

Compares the two most recent profiles and shows:
- Tables added/removed
- Row count changes with percentages
- Columns added/removed
- Column type changes, null rate drift, uniqueness changes
- Min/max value shifts, pattern changes

Includes text and JSON output formats.

24 unit tests covering: identical profiles, table add/remove,
row count changes, column add/remove, type changes, null drift,
uniqueness loss, value range shifts, pattern changes, edge cases,
text formatting, JSON formatting.

README: added diff command to commands table.
Tracker: updated Phase 2 progress, test counts (303 total).

Author: vahid110

README.md

@@ -62,6 +62,8 @@ Every finding includes:
 | `dqlens run --focus high` | Only HIGH severity findings |
 | `dqlens run --ci` | Exit code 1 on failure (for CI/CD) |
 | `dqlens run --json-output` | Output as JSON |
+| `dqlens diff` | Compare two most recent profiles |
+| `dqlens diff --json-output` | Diff as JSON |
 | `dqlens ignore <key>` | Suppress a known finding |
 
 ## Python API

src/dqlens/cli.py

@@ -4,6 +4,7 @@
     dqlens init      — Initialize .dqlens/ directory with connection config
     dqlens profile   — Profile database tables and save baseline
     dqlens run       — Run tests against live database, show problems
+    dqlens diff      — Compare two profiles to see what changed
     dqlens ignore    — Suppress a known finding
 """
 
@@ -301,6 +302,44 @@ def run(
         sys.exit(1)
 
 
+@main.command()
+@click.option("--json-output", "json_out", is_flag=True, help="Output diff as JSON.")
+def diff(json_out: bool):
+    """Compare the two most recent profiles to see what changed.
+
+    Shows schema changes (tables/columns added/removed), row count
+    shifts, null rate drift, uniqueness changes, and value range shifts.
+
+    Examples:
+        dqlens diff
+        dqlens diff --json-output
+    """
+    from dqlens.baseline import load_latest_profile, load_previous_profile
+    from dqlens.diff import diff_profiles, format_diff_json, format_diff_text
+
+    latest = load_latest_profile()
+    previous = load_previous_profile()
+
+    if latest is None:
+        click.echo("Error: No profiles found. Run 'dqlens profile' first.", err=True)
+        sys.exit(1)
+
+    if previous is None:
+        click.echo(
+            "Error: Only one profile found. Run 'dqlens profile' again to "
+            "create a second profile for comparison.",
+            err=True,
+        )
+        sys.exit(1)
+
+    result = diff_profiles(before=previous, after=latest)
+
+    if json_out:
+        print(json.dumps(format_diff_json(result), indent=2))
+    else:
+        click.echo(format_diff_text(result))
+
+
 @main.command()
 @click.argument("key")
 def ignore(key: str):