docs: complete alpha GDD package

[JOY (JOY)]

Summary

• Add alpha economy, balance, and playtest plan for TIME, SECOND, rewards, reinhabitation, metrics, and guardrails.
• Add alpha art, audio, and content direction for visual tone, role silhouettes, UI status colors, audio placeholders, and dialogue/content tone.
• Wire the completed alpha GDD package into the main GDD, vertical-slice spec, systems index, GitBook summary, roadmap, and changelog.

Local review

• Local code-review fallback: APPROVED for docs-only change.
• ADR/Hard Rule check: no backend/auth stack changes, no Unity asset or scene edits, no LLM authority expansion, and economy rules stay server-owned.

Verification

• npx markdownlint-cli2 "docs/design/26-alpha-game-design-document.md" "docs/design/27-alpha-level-design.md" "docs/design/29-alpha-questline-and-encounter-design.md" "docs/design/30-alpha-ux-flow.md" "docs/design/31-alpha-economy-balance-and-playtest-plan.md" "docs/design/32-alpha-art-audio-and-content-direction.md" "docs/design/12-game-design-document.md" "docs/design/03-systems-index.md" "docs/design/02-vertical-slice-spec.md" "docs/SUMMARY.md" "ROADMAP.md" "CHANGELOG.md"
• Local markdown link resolver: all local markdown links resolve.
• Em-dash scan: clean.
• git diff --cached --check