docs: add alpha questline and ux flow

[JOY (JOY)]

Summary

• Add the detailed alpha questline and encounter design for Check The Ash Underpass Signal.
• Define focus cast usage, quest steps, mission card fields, basic hostile roles, Tollkeeper Shell phases, rewards, failure states, and server-owned event ids.
• Add the alpha UX flow for HUD, NPC dialogue, Body Hall, Yard facility panel, Mission Board, Gate HUD, outcome report, offline-agent report, and reinhabitation.
• Update the Alpha GDD, Alpha Level Design, living GDD, systems index, vertical slice spec, GitBook summary, roadmap, and changelog to point at the new docs.

Local Review

• Local code-review fallback: APPROVED for docs/design lane. No engine code touched, no ADR violation found, and all quest, reward, combat, body, TIME, SECOND, and LLM boundaries remain server-owned.
• Markdown lint: pass.
• Diff hygiene: pass.
• Local markdown link check: pass.
• Em-dash scan on changed docs: pass.

Testing

• 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/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"
• git diff --cached --check
• local markdown link resolver over changed docs
• rg -n "—|–|…" over changed docs

Notes

This is documentation-only. Unity Play Mode smoke is not applicable for this PR.