docs: add soul skill carryover design

[JOY (JOY)]

Summary

• Add the soul skill carryover and sync design contract.
• Define how learned soul skills, body imprint, physical body capability, gear, soul-body sync, and server validation interact after reinhabitation.
• Link the new design doc from the GDD, alpha GDD, vertical slice spec, systems index, decision register, stats/skills doc, roadmap, changelog, and GitBook summary.

Local review

• Local code-review fallback: APPROVED. Docs-only change; no backend/auth/social stack change; no Unity client model calls; no LLM authority expansion; carryover, sync, and effective execution stay server-owned and validated by Nakama/Fusion.

Verification

• npx.cmd markdownlint-cli2 "docs/design/12-game-design-document.md" "docs/design/14-character-stat-and-relationship-system.md" "docs/design/26-alpha-game-design-document.md" "docs/design/38-alpha-design-decision-register.md" "docs/design/39-alpha-stats-and-skills-design.md" "docs/design/40-soul-skill-carryover-and-sync.md" "docs/design/03-systems-index.md" "docs/design/02-vertical-slice-spec.md" "docs/SUMMARY.md" "ROADMAP.md" "CHANGELOG.md"
• Markdown link resolver for changed docs: pass
• rg -n "—|–|…" ...changed docs...: no matches
• git diff --check
• git diff --cached --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reconcile carryover_rule values across design contracts

The new data contract introduces carryover_rule values (full, partial, locked_until_capable, body_only, etc.) that conflict with the existing alpha contract in docs/design/39-alpha-stats-and-skills-design.md (none, partial, full, or explicit rule id). Because both docs are now referenced as implementation sources, this creates an ambiguous enum for backend and Unity DTO/schema work, which can lead to incompatible serialization and validation rules depending on which document a developer follows.

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