fix: keep character target stable across sprite swaps

[han4wluc]

Summary

• stabilize character container ids by character.items[].id for normal authored characters so sprite-part swaps stay on one target
• preserve legacy indexed ids when duplicate character ids are authored, and add render-state regression coverage for both paths
• add dedicated grayscale sprite-part VT assets/spec coverage and bump the patch version to 0.7.6

Verification

• bun run vitest run puty.spec.js --testNamePattern addCharacters
• bun run vt:generate
• docker run --rm --user $(id -u):$(id -g) -e RTGL_VT_DEBUG=true -v "$PWD:/app" -w /app docker.io/han4wluc/rtgl:playwright-v1.57.0-rtgl-v1.0.10 rtgl vt screenshot --wait-event vt:ready --concurrency 1 --timeout 60000 --item character/sprite-parts-update.yaml --item character/duplicate-character.yaml
• rtgl vt report --diff-threshold 0.8 --item character/sprite-parts-update.yaml --item character/duplicate-character.yaml
• manual served-page sanity check with playwright-cli open --browser=chromium http://localhost:3004/candidate/character/sprite-parts-update.html and playwright-cli screenshot