docs: define milestone v1.3 requirements

.planning/REQUIREMENTS.md

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+# Requirements: GearBox v1.3 Research & Decision Tools
+
+**Defined:** 2026-03-16
+**Core Value:** Make it effortless to manage gear and plan new purchases -- see how a potential buy affects your total setup weight and cost before committing.
+
+## v1.3 Requirements
+
+Requirements for this milestone. Each maps to roadmap phases.
+
+### Comparison View
+
+- [ ] **COMP-01**: User can view candidates side-by-side in a tabular comparison layout (weight, price, images, notes, links, status)
+- [ ] **COMP-02**: User can see relative deltas highlighting the lightest and cheapest candidate with +/- differences
+- [ ] **COMP-03**: Comparison table scrolls horizontally with a sticky label column on narrow viewports
+- [ ] **COMP-04**: Comparison view displays read-only summary for resolved threads
+
+### Candidate Ranking
+
+- [ ] **RANK-01**: User can drag candidates to reorder priority ranking within a thread
+- [ ] **RANK-02**: Top 3 ranked candidates display rank badges (gold, silver, bronze)
+- [ ] **RANK-03**: User can add pros and cons text per candidate displayed as bullet lists
+- [ ] **RANK-04**: Candidate rank order persists across sessions
+- [ ] **RANK-05**: Drag handles and ranking are disabled on resolved threads
+
+### Impact Preview
+
+- [ ] **IMPC-01**: User can select a setup and see weight and cost delta for each candidate
+- [ ] **IMPC-02**: Impact preview auto-detects replace mode when a setup item exists in the same category as the thread
+- [ ] **IMPC-03**: Impact preview shows add mode (pure addition) when no category match exists in the selected setup
+- [ ] **IMPC-04**: Candidates with missing weight data show a clear indicator instead of misleading zero deltas
+
+## Future Requirements
+
+Deferred to future milestones. Tracked but not in current roadmap.
+
+### Data Management
+
+- **DATA-01**: User can import gear collection from CSV
+- **DATA-02**: User can export gear collection to CSV
+
+### Social & Multi-User
+
+- **SOCL-01**: User can create an account with authentication
+- **SOCL-02**: User can share collections and setups publicly
+- **SOCL-03**: User can view other users' public profiles and setups
+
+### Automation
+
+- **AUTO-01**: System can auto-fill product information (price, weight, images) from external sources
+
+## Out of Scope
+
+Explicitly excluded. Documented to prevent scope creep.
+
+| Feature | Reason |
+|---------|--------|
+| Custom comparison attributes | Complexity trap -- weight/price covers 80% of cases |
+| Score/rating calculation | Opaque algorithms distrust; manual ranking expresses user preference better |
+| Cross-thread comparison | Candidates are decision-scoped; different categories are not apples-to-apples |
+| Classification-aware impact breakdown | Data available but UI complexity high; flat delta covers 90% of use case |
+| Comparison permalink | Requires auth/multi-user work not in scope for v1 |
+| Mobile-optimized comparison (swipe) | Horizontal scroll works for now |
+| Rank badge on card grid view | Low urgency; add when users express confusion |
+
+## Traceability
+
+Which phases cover which requirements. Updated during roadmap creation.
+
+| Requirement | Phase | Status |
+|-------------|-------|--------|
+| COMP-01 | — | Pending |
+| COMP-02 | — | Pending |
+| COMP-03 | — | Pending |
+| COMP-04 | — | Pending |
+| RANK-01 | — | Pending |
+| RANK-02 | — | Pending |
+| RANK-03 | — | Pending |
+| RANK-04 | — | Pending |
+| RANK-05 | — | Pending |
+| IMPC-01 | — | Pending |
+| IMPC-02 | — | Pending |
+| IMPC-03 | — | Pending |
+| IMPC-04 | — | Pending |
+
+**Coverage:**
+- v1.3 requirements: 13 total
+- Mapped to phases: 0
+- Unmapped: 13
+
+---
+*Requirements defined: 2026-03-16*
+*Last updated: 2026-03-16*