Add dedicated overview-ruler annotation types for unified diff

[tobiasmelcher]

Closes #2941

What

Adds overview-ruler markers for top-level unified-diff hunks. Until now the
unified diff presentation highlighted differences only in the visible editor
area and showed nothing on the overview ruler, so off-screen differences were
hard to discover and navigate.

This is done by introducing two dedicated overview-ruler annotation types:

• org.eclipse.compare.unifieddiff.internal.addition.ruler
• org.eclipse.compare.unifieddiff.internal.deletion.ruler

They paint only on the overview ruler, with saturated, high-contrast
colors (40,140,40 green and 220,50,50 red) and a sharp 1px border. Each
ruler annotation also carries a human-readable tooltip ("Addition at line N",
"Deletion from line M to N") shown when hovering a ruler tick. The existing
addition / deletion types continue to paint the pastel text highlight in
the editor and are untouched. Detailed/token-level diff annotations
(detailedAddition, detailedDeletion) deliberately produce no ruler
markers.

Why two new types instead of reusing the existing annotations

In Eclipse, an annotation type declared in markerAnnotationSpecification
uses a single colorPreferenceValue for both the editor text highlight
(drawn by AnnotationPainter) and the overview ruler (drawn by
OverviewRuler).

• For the text highlight to stay readable behind source code, the diff colors
  must be light pastels (229,242,229 for additions, 255,229,229 for
  deletions).
• OverviewRuler computes its fill by fading the color 75% towards the
  background, and its 1px border by fading it 20% towards the background. With
  those pastels, both the fill and the border become virtually invisible on
  the ruler.
• Overriding the ruler globally with setUseSaturatedColors(true) is not an
  option: it is global and would ruin the saturation of every other annotation
  in the editor (warnings, search results, bookmarks, etc.).

Dedicated ruler-only annotation types let each side use the color it needs:
pastel for a readable in-text highlight, saturated for a clearly visible ruler
tick, without touching any other annotation in the workbench.

Notes

• The unused symbolicIcon="info" is dropped from the four pre-existing
  inline-diff specs. The icon was never shown because all four set
  verticalRulerPreferenceValue and contributesToHeader to false.
• No dark-theme color override is set for the new ruler types, by intention.
  The standard error and warning overview-ruler annotations also set no
  dark-theme override in org.eclipse.ui.themes — they use their
  colorPreferenceValue defaults unchanged in both themes. The new
  addition.ruler / deletion.ruler types follow the same convention for
  symmetry.

---

This change was prepared with the help of Claude Code.

[github-actions]

Test Results

    51 files  ± 0      51 suites  ±0   58m 6s ⏱️ +45s
 4 480 tests + 4   4 457 ✅ + 3   22 💤 ±0  1 ❌ +1 
11 529 runs  +12  11 374 ✅ +11  154 💤 ±0  1 ❌ +1 

For more details on these failures, see this check.

Results for commit 20e5e87. ± Comparison against base commit 8b229d0.

♻️ This comment has been updated with latest results.