feat: add JSDoc + checkJs TypeScript type checking (Option B)

[Copilot]

Enables TypeScript's type checker on the existing .js source without a build step or syntax migration — the browser still loads the original files directly.

New files

• tsconfig.json — checkJs: true, strict: true, noEmit: true; scope limited to death-clock-core.js and scripts/*.js
• global.d.ts — extends Window with the project's dual-export globals (MilestonesData, DeathClockCore, ChangelogData, ProjectStatsData) and declares the Milestone / ChangelogRelease interfaces

Bugs surfaced and fixed by the type checker

• Date arithmetic — d - base replaced with d.getTime() - base.getTime() in generateProjectionData and getTimeDelta; TypeScript correctly rejects implicit Date subtraction
• Index access on Object — computePassiveRate params tightened from {Object} to {Object.<string, number>} / {Object.<string, boolean>} so agents[a.id] and roles[r.id] are safe
• never[] inference in build-changelog — currentRelease.sections and currentSection.items were inferred as never[], silently making push() a no-op for TypeScript; fixed with explicit @typedef + @type annotations
• Untyped yaml.load() results — doc in build-milestones.js and build-project-stats.js cast to typed shapes so property access is checked

JSDoc annotation improvements

• {Array} → {Milestone[]} / {Milestone|null} in getTriggeredMilestones, getNextMilestone, getNextMilestoneForPlayer — callback access to .tokens is now statically verified
• Helper functions in build scripts annotated with @param / @typedef

CI

npm run typecheck (tsc --noEmit) added as a step in unit-tests.yml, running before Jest, so type regressions fail the PR.

[github-actions]

👁️ PR Preview

🚀 Open Preview

Deployed from commit 6919bf1 · Updates on every push to this PR
(Preview is removed automatically when the PR is closed.)

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3cd91d7) to head (6919bf1).
⚠️ Report is 1 commits behind head on main.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #54   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          233       233           
  Branches       107       107           
=========================================
  Hits           233       233           

Flag	Coverage Δ
unit	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
death-clock-core.js	100.00% <100.00%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3cd91d7...6919bf1. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.