Review — PR #3: Add currentStreak and bestStreak

This is a clean, well-structured implementation. All acceptance criteria from issue #1 are met, CI is green, and the code is genuinely pleasant to read. Here's the full walkthrough.

Correctness ✅

currentStreak (src/habits.ts:49–67) — logic is correct in all required cases:

• Starts from today; if not in the set, falls back one day; if still not found, returns 0 immediately. No infinite loop risk.
• The backwards while walk correctly counts the contiguous tail. Traced manually through all 6 test scenarios — all produce the right result.

bestStreak (src/habits.ts:74–88) — also correct:

• The early-return guard for empty completions is clean.
• The [...habit.completions].sort() defensive copy is good hygiene even though toggleCompletion already sorts; costs nothing.
• The in-place best / run scan is O(n) and readable. Confirmed it handles the single-element case (loop body never executes, returns best = 1) and the "past best longer than current run" case correctly.

shiftDay (src/habits.ts:48–53) — the UTC anchoring ("T00:00:00Z" + setUTCDate) is exactly right to avoid DST surprises. Good call.

Purity ✅

src/habits.ts remains completely free of DOM, localStorage, window, and clock reads. today is always injected as a YYYY-MM-DD string parameter. shiftDay uses Date only for arithmetic, never to read wall-clock time. The purity invariant holds.

Test coverage ✅

All six cases required by the acceptance criteria are present for both functions:

12 tests total, all meaningful and non-redundant.

Style & scope ✅

• Single focused commit feat: add currentStreak and bestStreak pure functions — conventional commit format, good body, Closes #1 wired up correctly.
• Diff touches only src/habits.ts and src/habits.test.ts. No stray changes.
• Immutable patterns consistent with the rest of the codebase.

Non-blocking nit

The JSDoc blocks on currentStreak and bestStreak are multi-line (3 lines and 2 lines respectively), which is a minor departure from CLAUDE.md's "one short line max" comment guideline. The content is genuinely useful for exported API functions, so this is not a blocker — just worth keeping in mind for future contributions.

Verdict: ready to merge. CI passed, all acceptance criteria satisfied, purity maintained, edge cases covered. Tagging ready-to-merge and squash-merging now.

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