docs(07-01): complete weight unit core plan

- SUMMARY.md with TDD results and 21-test coverage
- STATE.md updated to Plan 2 of 2, 17% progress
- ROADMAP.md marks 07-01 complete (1/2 plans)
- REQUIREMENTS.md marks UNIT-02, UNIT-03 complete

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.planning/phases/07-weight-unit-selection/07-01-SUMMARY.md

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+---
+phase: 07-weight-unit-selection
+plan: 01
+subsystem: ui
+tags: [weight-conversion, formatters, react-hooks, tdd]
+
+# Dependency graph
+requires: []
+provides:
+  - "WeightUnit type export for all weight display components"
+  - "Parameterized formatWeight(grams, unit) with g/oz/lb/kg support"
+  - "useWeightUnit() hook wrapping settings API for typed unit access"
+affects: [07-02-PLAN]
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns: [unit-conversion-via-formatters, settings-backed-hooks]
+
+key-files:
+  created:
+    - src/client/hooks/useWeightUnit.ts
+    - tests/lib/formatters.test.ts
+  modified:
+    - src/client/lib/formatters.ts
+
+key-decisions:
+  - "Conversion precision: g=0dp, oz=1dp, lb=2dp, kg=2dp matching common usage"
+  - "useWeightUnit validates stored value against known units to protect against corrupt data"
+
+patterns-established:
+  - "Weight formatting: always call formatWeight(grams, unit) with WeightUnit parameter"
+  - "Settings-backed hooks: wrap useSetting with typed validation for domain-specific config"
+
+requirements-completed: [UNIT-02, UNIT-03]
+
+# Metrics
+duration: 2min
+completed: 2026-03-16
+---
+
+# Phase 7 Plan 01: Weight Unit Core Summary
+
+**Parameterized formatWeight with g/oz/lb/kg conversion and useWeightUnit settings hook, backed by 21 TDD tests**
+
+## Performance
+
+- **Duration:** 2 min
+- **Started:** 2026-03-16T11:14:19Z
+- **Completed:** 2026-03-16T11:16:30Z
+- **Tasks:** 2
+- **Files modified:** 3
+
+## Accomplishments
+- TDD-developed formatWeight function supporting 4 weight units (g, oz, lb, kg) with appropriate precision
+- WeightUnit type exported for consumption by all display components in Plan 02
+- useWeightUnit convenience hook with validation and "g" default, ready for component integration
+- Full backward compatibility preserved -- formatWeight(grams) still returns "Xg" as before
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1 (RED): TDD formatWeight tests** - `431c179` (test)
+2. **Task 1 (GREEN): Implement formatWeight with unit parameter** - `6cac0a3` (feat)
+3. **Task 2: Create useWeightUnit convenience hook** - `ada3791` (feat)
+
+_TDD task had 2 commits (test -> feat). No refactor needed -- code was already minimal._
+
+## Files Created/Modified
+- `src/client/lib/formatters.ts` - Added WeightUnit type, conversion constants, switch-based unit formatting
+- `src/client/hooks/useWeightUnit.ts` - Convenience hook wrapping useSetting("weightUnit") with typed validation
+- `tests/lib/formatters.test.ts` - 21 tests covering all units, null/undefined, backward compat, edge cases
+
+## Decisions Made
+- Conversion precision follows common usage: grams rounded (0dp), ounces 1dp, pounds 2dp, kilograms 2dp
+- useWeightUnit validates stored value against a whitelist of known units, protecting against corrupt settings data
+- Conversion constants (GRAMS_PER_OZ=28.3495, GRAMS_PER_LB=453.592, GRAMS_PER_KG=1000) kept as module-level consts, not exported
+
+## Deviations from Plan
+
+### Auto-fixed Issues
+
+**1. [Rule 1 - Bug] Fixed import order in useWeightUnit.ts**
+- **Found during:** Task 2 (useWeightUnit hook creation)
+- **Issue:** Biome lint required imports sorted alphabetically (type imports before value imports)
+- **Fix:** Reordered imports to put `import type { WeightUnit }` before `import { useSetting }`
+- **Files modified:** src/client/hooks/useWeightUnit.ts
+- **Verification:** `bun run lint` passes clean
+- **Committed in:** ada3791 (Task 2 commit)
+
+---
+
+**Total deviations:** 1 auto-fixed (1 bug - lint order)
+**Impact on plan:** Trivial formatting fix. No scope creep.
+
+## Issues Encountered
+None
+
+## User Setup Required
+None - no external service configuration required.
+
+## Next Phase Readiness
+- WeightUnit type and formatWeight function ready for Plan 02 to wire into all weight-displaying components
+- useWeightUnit hook ready for components to consume the user's preferred unit from settings
+- All 108 existing tests pass (full suite regression check confirmed)
+
+## Self-Check: PASSED
+
+All files exist, all commits found, all exports verified.
+
+---
+*Phase: 07-weight-unit-selection*
+*Completed: 2026-03-16*