chore(spec): auto-polish scatter-regression-linear (#10131)

Automated spec polish from `daily-regen` pre-flight.

**Spec:** `scatter-regression-linear`

## What changed
- Feature tag: `trendline` → `trend` (canonical form from features
table)
- Updated timestamp bumped to current UTC

## Why
The `trendline` tag is well-formed but not in the canonical features
table. The advisory table lists `trend` as the standard feature
descriptor for trend visualization. Canonicalization prefers the
established term when one exists.

## Hard guarantees from the prompt
- `id`, `issue`, `created` unchanged
- No semantic changes (data shape, plot type, requirements identical)
- `updated` bumped to current UTC

Auto-merge enabled — will merge once required checks pass. The skip-gate
in `daily-regen` prevents stacking polish PRs for this spec.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

plots/scatter-regression-linear/specification.yaml

@@ -6,7 +6,7 @@ title: Scatter Plot with Linear Regression
 
 # Specification tracking
 created: 2025-12-24T21:21:30Z
-updated: 2026-05-06T22:22:58Z
+updated: 2026-08-05T12:59:02Z
 issue: 1821
 suggested: MarkusNeusinger
 
@@ -26,4 +26,4 @@ tags:
     - regression
     - confidence-interval
     - correlation
-    - trendline
+    - trend