Post-MVP hardening: code-review cleanup, CI, and next-steps plan

[TimBest]

Stacked on top of the MVP work (PR #12). This PR isolates the three post-MVP commits so they can be reviewed independently.

Base branch is claude/commodity-visualizer-base (the MVP + Netlify baseline) rather than master, so the diff shows exactly these three commits. master is still the legacy Rust/Python code — CI can't run there. Once PR #12 lands, this PR can be retargeted to master.

Commits

1. refactor: code review cleanup — applied findings from a three-agent review:
   • Extracted formatMeters/formatBigUSD into lib/formatting.ts (+ tests)
   • Added isPositiveFinite type guard in lib/math.ts, reused in urlState.ts and FallbackForm.tsx
   • Split IsoViz into focused subcomponents; capped stacked-glyph DOM at 20 nodes
   • Added cache-control headers to /api/geocode and /api/footprint
   • Centralized URL building in clientFetchers via URLSearchParams
2. docs: next-steps plan — docs/NEXT_STEPS.md: MVP status snapshot and an ordered P0–P5 plan to finish the spec.
3. ci: GitHub Actions workflow — .github/workflows/ci.yml: runs bun install --frozen-lockfile, bun run typecheck, bun test, bun run build on every push to master and every PR. Pins Bun 1.3.11; cancels superseded runs via concurrency groups.

Test plan

• bun test — 78/78 pass
• bun run typecheck — clean
• bun run build — clean
• CI workflow turns green on this PR (first live run)

Note

These three commits also exist on PR #12's branch (claude/commodity-visualizer-display-netlify) — they were left there intentionally. Whichever PR merges first, git will no-op the duplicates in the other.

https://claude.ai/code/session_01Df7Y187BB8uPFqRxkrJbiz

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