docs: add alpha visual budget

[JOY (JOY)]

Summary

• add docs/design/42-alpha-visual-budget.md for issue [Alpha Art] Define Web and mobile visual budget for alpha #177
• define PC native alpha, Web demo, and mobile candidate visual budgets
• cover character, environment, material, lighting, NPC count, VFX, audio, and WebGL risks
• link the budget from the art direction, asset shortlist, systems index, GitBook summary, roadmap, and changelog

Local review

Local code-review fallback: APPROVED for docs-only visual budget scope.

Checked manually:

• no Unity scene, prefab, package, backend, or authoritative gameplay changes
• no Web build implementation or mobile commitment is introduced
• budget explicitly links WebGL risk back to issue Create WebGL demo build lane for external playtesters #151
• issue [Alpha Art] Define Web and mobile visual budget for alpha #177 acceptance criteria are covered

Verification

• npx.cmd markdownlint-cli2 "docs/**/*.md" "*.md"
• git diff --check
• local markdown link resolver for changed docs
• em-dash scan on changed docs

Closes #177