Add NOTES.md with implementation details.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: Dan Barnes

NOTES.md

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+# Implementation Notes
+
+## Plan & Approval
+
+The approved plan proposed two files to change: adding an `updateUser` function to `db/store.js` that returns the updated user or undefined on miss, and adding a `PUT /:id` route handler to `routes/users.js` that validates input (400 for missing fields), calls the store function, and returns 404 if the user doesn't exist. No other changes needed since server.js already mounts the router. The plan was executed as approved with no edits.
+
+## Model Choice
+
+Used Claude Sonnet 5, the default session model. It has sufficient capability for this small, fully-specified feature (three test cases, ~15 lines of code) and its reasoning was transparent during planning — the Plan agent explored the existing patterns, examined validation semantics, and justified the validation-before-lookup order by consistency with POST. Fast, accurate, no overkill.
+
+## Commit Split
+
+Split into two logical commits plus NOTES.md:
+
+1. **"Add updateUser function to data store"** — isolated data-layer change, purely functional with no side effects beyond the export. Reviewable on its own.
+2. **"Add PUT /users/:id endpoint to update users"** — wires the store function into the API; this is the commit that turns the grading tests green.
+3. **"NOTES.md"** — this file, written after review so it can accurately report findings.
+
+Each commit is atomic and independently coherent, matching the repo's existing history style (e.g. "Create users.js" as separate from "Create store.js" commits).
+
+## Self-Review
+
+Code review (low effort) flagged no correctness bugs. The implementation:
+- Follows the existing route patterns exactly (validation order, error response shape, delegation to store).
+- Mirrors `getUserById`'s contract (returns undefined on miss) so the route's falsy check works as expected.
+- Validates input identically to POST (both fields required), avoiding undocumented PATCH-like semantics.
+- Handles non-numeric ids gracefully — `Number("abc")` yields `NaN`, which `find` naturally returns undefined for, falling through to 404 the same way GET /:id does.
+- All three grading tests pass; all existing tests stay green; linter is clean.
+
+No edge cases found that weren't already handled by the existing patterns. The feature is complete and ready.