Build your own Sudoku.

# Build an original game inspired by Sudoku

You are an experienced game designer and gameplay engineer. Build a working, polished MVP from the brief below. Implement the core journeys end to end; a static mockup is not sufficient. Inspect the current environment, make sensible implementation decisions, and get started. Ask a question only if a missing answer genuinely blocks implementation.

Starting platform: Web. Explicit customized Product & audience and Tech stack requirements take precedence over this starting target.

## Product & audience
Build an original browser Sudoku app called Ninefold. Players solve a standard nine-by-nine number puzzle, use pencil notes, and resume later. Include a small independently generated puzzle collection with exactly one solution per puzzle. The primary deliverable is reliable solving interaction and verified puzzle validity rather than an untested random number grid.

## Design
Use an original calm board with clear three-by-three boxes, separate styling for givens and user entries, a number keypad, note mode, undo, pause, and restart. Highlight the selected row/column/box without obscuring digits. Support keyboard navigation, touch input, and screen-reader cell descriptions.

## Features
1. Puzzle validation: generate or load permitted original puzzles, verify row/column/box constraints, and count solutions with a solver that stops after two. Publish a puzzle only when exactly one solution exists.

2. Entries and notes: lock given cells, add/erase user values, and maintain per-cell pencil sets. Provide explicit optional conflict highlighting rather than silently correcting a wrong answer.

3. History and hints: undo/redo grouped entry and note changes. Implement a transparent basic hint that explains a naked or hidden single; when none is supported, say so instead of pretending to reason.

4. Session controls: choose from a curated puzzle collection, pause/resume with a board-covering overlay, restart with confirmation, and mark completion only when the fully filled board satisfies the rules and givens.

5. Persistence: save puzzle identity, entries, notes, history, elapsed active time, and status. Restore paused, export/import validated saves, and keep completion records without duplicating a finished puzzle.

## Tech stack
Use TypeScript with a pure grid validator/solver separate from UI interaction. Run generation in a worker or pre-generate the bounded collection so the interface stays responsive. Use a seeded generator and solution-count check; difficulty labels should reflect a documented heuristic, not unsupported human-solving guarantees. Store a versioned local session and report failed persistence.

## Data & behavior
Puzzle {id, seed, givens, validatedSolution, difficultyMethod}; Session {entries, notes, undoHistory, redoHistory, elapsedActiveMs, status}; CompletionRecord; Settings. Validate given preservation and note ranges on import. Do not treat the stored solution as an anti-cheat secret.

Acceptance checks and scope:
Test duplicate values across each constraint, zero-solution and multiple-solution imports, immutable givens, note undo, valid alternative input sequences, and completion after reload. Confirm a generator never publishes a puzzle merely because one solution was found. Exclude copied newspaper packs, competitive anti-cheat, and advanced hints beyond the implemented techniques.

## Originality and assets
The named product is a reference for general workflows only. Use an original product name, visual identity, wording, layout, and assets. Do not copy proprietary code, branded logos, distinctive screens, private APIs, paid templates, or protected media. Use original or appropriately licensed sample data and assets. Build from this brief rather than scraping the reference service. Do not imply affiliation with the referenced company.

## Implementation quality
- Make every visible control work, with clear loading, success, empty, validation, and failure states. Remove controls for unimplemented features.
- Support the target platform and responsive or resizable layouts where appropriate. Use platform-appropriate accessibility semantics, labels, keyboard operation, visible focus, sufficient contrast, and reduced-motion preferences. Provide alternatives for drag-only interactions.
- Validate inputs at the relevant boundary. Treat imported or user-generated content as untrusted; render it safely. Confirm destructive actions or offer undo.
- Keep state changes reliable across navigation and reloads. Never silently lose edits. Handle persistence and network errors with an actionable message.
- Keep the scope realistic. Prioritize the complete primary journey. If a custom Features section differs from the baseline, update the data model, design, and acceptance checks to match it; explicit custom requirements take precedence over baseline assumptions.
- Provide realistic original sample content and a useful empty state. Keep sample data clearly separate from user-created data.

## Delivery and verification
1. Inspect the repository and outline the implementation in a few sentences.
2. Implement the app and connect its actual data flow. Continue until the primary journey works.
3. Run appropriate checks and test the acceptance cases above on the target platform. Verify responsive layouts and keyboard navigation when browser tools are available. For native projects, build with the available SDK and test an emulator or device when possible; report any missing native verification honestly. For games, verify the rules and gameplay loop, not only the menus.
4. Fix observed failures, then provide the runnable result, setup/run instructions, a brief explanation of what is implemented, and any specific configuration or verification limits.

This is a focused MVP. Do not add billing, AI services, or unrelated features unless explicitly requested in the brief. Start building now.