fix: harden prototype combat target lifecycle

[JOY (JOY)]

Summary

• prune destroyed prototype combat target references from the active target registry
• make the training target spawner recreate a missing target while still treating live or rebuilding targets as present
• update roadmap, changelog, and Play Mode smoke checklist for target lifecycle coverage

Verification

• npx.cmd markdownlint-cli2@0.15.0 CHANGELOG.md ROADMAP.md docs/setup/play-mode-smoke-checklist.md
• rg -n "—|–" CHANGELOG.md ROADMAP.md docs/setup/play-mode-smoke-checklist.md Unity/Assets/_SecondSpawn/Scripts/AI/PrototypeCombatTarget.cs Unity/Assets/_SecondSpawn/Scripts/AI/PrototypeCombatTrainingTargetSpawner.cs (no matches)
• git diff --check
• local code-review fallback: APPROVED, Unity-only prototype helper change with no authority, reward, package, or provider-key boundary issues

Notes

Root Unity recompile and Play Mode smoke will be run after merge into dev because the active Unity Editor is opened on the root project.

Closes #148