docs: define v1 requirements

.planning/REQUIREMENTS.md

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+# Requirements: Expense Splitter
+
+**Defined:** 2026-03-11
+**Core Value:** Everyone leaves knowing exactly what they owe - no arguing, no awkward math at the table.
+
+## v1 Requirements
+
+Requirements for initial release. Each maps to roadmap phases.
+
+### People & Items
+
+- [ ] **PEOPLE-01**: User can add people to the bill by name
+- [ ] **PEOPLE-02**: User can add items with name and price
+- [ ] **PEOPLE-03**: User can assign items to specific people
+- [ ] **PEOPLE-04**: User can mark items as "shared" to split across selected people
+
+### Calculations
+
+- [ ] **CALC-01**: System calculates per-person subtotal from assigned items
+- [ ] **CALC-02**: User can set a global tip percentage
+- [ ] **CALC-03**: System applies tip to calculate final per-person total
+- [ ] **CALC-04**: System displays clear breakdown showing who owes what
+
+### Persistence
+
+- [ ] **PERSIST-01**: System saves bills to browser local storage
+- [ ] **PERSIST-02**: User can view history of past bills
+- [ ] **PERSIST-03**: User can load a past bill to review
+
+## v2 Requirements
+
+Deferred to future release. Tracked but not in current roadmap.
+
+### Enhanced Features
+
+- **PEOPLE-05**: User can edit/delete people and items after adding
+- **PEOPLE-06**: User can use quick equal split option for simple bills
+- **PEOPLE-07**: User can split by percentage (e.g., someone pays 60%)
+- **CALC-05**: User can set custom tip percentage per person
+- **CALC-06**: System handles tax with multiple split options (even, proportional)
+- **PERSIST-04**: User can export summary as text or image
+
+## Out of Scope
+
+Explicitly excluded. Documented to prevent scope creep.
+
+| Feature | Reason |
+|---------|--------|
+| User accounts/authentication | Local storage only - no backend complexity |
+| Backend server | Fully client-side app |
+| Mobile native apps | Web-first, works in mobile browser |
+| Receipt scanning/OCR | Manual entry is faster than correcting bad OCR |
+| Payment processing | Calculation only - no money transfer |
+| Bill sharing via link | Local storage is device-specific |
+| Real-time collaboration | Single-device entry (one person enters all) |
+
+## Traceability
+
+Which phases cover which requirements. Updated during roadmap creation.
+
+| Requirement | Phase | Status |
+|-------------|-------|--------|
+| PEOPLE-01 | Phase 1 | Pending |
+| PEOPLE-02 | Phase 1 | Pending |
+| PEOPLE-03 | Phase 1 | Pending |
+| PEOPLE-04 | Phase 1 | Pending |
+| CALC-01 | Phase 1 | Pending |
+| CALC-02 | Phase 1 | Pending |
+| CALC-03 | Phase 1 | Pending |
+| CALC-04 | Phase 1 | Pending |
+| PERSIST-01 | Phase 2 | Pending |
+| PERSIST-02 | Phase 2 | Pending |
+| PERSIST-03 | Phase 2 | Pending |
+
+**Coverage:**
+- v1 requirements: 11 total
+- Mapped to phases: 11
+- Unmapped: 0 ✓
+
+---
+*Requirements defined: 2026-03-11*
+*Last updated: 2026-03-11 after initial definition*