docs: define alpha level and stat progression

[JOY (JOY)]

Summary

• Add docs/design/57-alpha-level-core-stat-progression.md as the implementation packet for issue Define alpha level, XP, and core stat allocation packet #275.
• Lock alpha defaults: current-body level cap 15, 3 Body Points per level, no automatic plus-one-to-all-stat growth, fixed XP table, and body-bound stat point loss on reinhabitation.
• Link the packet from the systems index, GitBook nav, stat/skill docs, soul carryover docs, inventory docs, and ROADMAP.

Design stance

• Uses Solo Leveling-style stat allocation as a readability reference, but does not copy the five-points-per-level economy because SECOND SPAWN has eight core stats, body archetypes, body loss, and reinhabitation reset pressure.
• Pick Me Up-style roster risk informs the body-bound reset and survival pressure.

Verification

• tools\windows\check-backlog-sync.ps1
• npx.cmd --yes markdownlint-cli2 ROADMAP.md docs/SUMMARY.md docs/design/03-systems-index.md docs/design/14-character-stat-and-relationship-system.md docs/design/39-alpha-stats-and-skills-design.md docs/design/40-soul-skill-carryover-and-sync.md docs/design/45-inventory-and-equipment-system.md docs/design/57-alpha-level-core-stat-progression.md
• git diff --check
• rg -n "—|–" ROADMAP.md docs/SUMMARY.md docs/design/03-systems-index.md docs/design/14-character-stat-and-relationship-system.md docs/design/39-alpha-stats-and-skills-design.md docs/design/40-soul-skill-carryover-and-sync.md docs/design/45-inventory-and-equipment-system.md docs/design/57-alpha-level-core-stat-progression.md returned no matches.

References:

• https://progameguides.com/solo-leveling-arise/best-stats-to-add-for-sung-jinwoo-in-solo-leveling-arise/
• https://www.thegamer.com/solo-leveling-arise-best-stats-level-up-reset-guide/
• https://pick-me-up.fandom.com/wiki/Pick_Me_Up%21/Heroes

Closes #275.