docs(showcase): repair broken v2 asset paths in index.html

[DemchaAV]

Summary

External presentation audit ahead of the v1.6.6 Maven Central debut surfaced 20 of 27 showcase references in docs/index.html rendering as 404. After the CV v2 layered migration in 1.6.x, every preset's screenshot and PDF was renamed with a -v2 suffix on disk, but the GitHub Pages site (demchaav.github.io/GraphCompose/) still referenced the pre-migration paths.

Fix

Pattern	Action	Count
cv/cv-X.{pdf,png}	→ cv/cv-X-v2.{pdf,png}	14 CV PDFs + 1 PNG
coverletter/cover-letter-X.{pdf,png}	→ coverletter/cover-letter-X-v2.{pdf,png}	4 specific cover letters
coverletter/cover-letter.pdf	→ coverletter/cover-letter-modern-professional-v2.pdf	1 generic landing link
Invoice / proposal / schedule	unchanged	already correct

Verification

$ for ref in $(grep -oE 'showcase/[a-z]+/templates/[a-z/]+[a-z-]+(-v2)?\.[a-z]+' docs/index.html | sort -u); do
    [ -f "docs/$ref" ] || echo "BROKEN: $ref"
  done
(empty — all 26 unique references resolve on disk)

Why this is a release-blocker

The next Maven Central debut tag (v1.6.6) will route significantly more traffic at the Pages site. Broken-image-laden first impression for every new caller would have cost trust. Fixing this before cut is cheap; fixing after means days of broken homepage in the Central debut window.

This is Phase A of the pre-release polish plan; Phases B + C (hero PDF + README banner replacement) follow.

Test plan

• All 26 unique referenced paths resolve via ls
• Diff is mechanical / structurally identical (sed substitution only)
• CI green on PR
• Visual verification on Pages once merged (homepage hero + CV gallery)