Conform thread message types to the registry (location + battery)

[ikidnapmyself]

Summary

Makes the thread screen's message bodies conform to the Swinn message-type registry ({ type, version, payload }, payload validated against the type schema), replacing ad-hoc shapes. Minimal scope: location and battery; other types are out of scope.

• services/messageTypes.js — new pure builders buildLocation(lat, lng) and buildBatteryMetric(value) returning the exact canonical bodies, validating only what the registry schema/constraints require (lat/lng ranges; battery_level → percent).
• LocationComponent → posts { type: 'location', version: '1.0', payload: { lat, lng } } (was the raw expo-location object).
• BatteryComponent → posts { type: 'metric', version: '1.0', payload: { quantity: 'battery_level', value, unit: 'percent' } } (was { battery }).
• MessageScreen — removed the non-conforming "Random Numbers" test button and its now-dead imports.

Builders are the single choke point; both composers route through them under a try/catch, so a malformed reading never reaches the API. Renderer is intentionally untouched (still shows the canonical JSON) — renderer cards for all 7 types are a follow-up.

Design: docs/plans/2026-07-17-thread-message-types-design.md

Test Plan

• node services/messageTypes.test.cjs — valid location/metric deep-equal the canonical objects; out-of-range/non-number inputs throw
• All touched files parse under babel-preset-expo
• Drive the thread screen: tap Location + Battery, confirm POST bodies are canonical {type,version,payload} and the Random Numbers button is gone

Stacked on #45 (base: feature/expo-sdk-upgrade).

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