Bound measureText/wrapText caches (LRU) to stop unbounded growth

[fudianchn]

Summary

measureText() and wrapText() each keep a module-level cache with no eviction, keyed by the full text:

• src/measure-text.ts — const cache = new Map()
• src/wrap-text.ts — const cache = {}

Every distinct string an app ever renders is retained for the lifetime of the process, so any app whose text changes over time (a streaming/typing indicator, a growing log, a clock, a progress line) leaks monotonically until it OOMs (#986). Both caches are fed from the Yoga measure function on every layout pass, so entries accumulate even with renderThrottleMs.

Fix

Replace the unbounded Map/Record with a small bounded LRU cache (src/lru-cache.ts):

• capped (currently 1000 entries each — recomputation on eviction is cheap),
• refresh-on-read, so recently-used entries stay hot.

Normal apps (which measure/wrap a bounded set of strings) are unaffected; only ever-growing distinct-string sets are now bounded.

Verification

• npm run typecheck ✅ (ink's erasableSyntaxOnly tsconfig — note the helper uses an explicit field, not a constructor parameter property)
• npm run lint (xo) ✅
• ava ✅ — added test/lru-cache.ts (eviction + refresh-on-read + missing-key recency); existing measure-text/flex-wrap (which exercises wrapText) tests still pass.

This matches the LRU-cap approach validated in the issue (costajhnt: capping the caches flat-lines the retained heap).

Closes #986