perf(ink): replace cache.clear() with LRU eviction in line-width cache

[HUQIANTAO]

Summary

The lineWidth cache in packages/ink/src/line-width-cache.ts used cache.clear() when the Map reached 4096 entries, which discards ALL cached widths at once. This creates a sawtooth hit-rate pattern:

1. Cache slowly fills up with computed widths
2. At 4096 entries → cache.clear() drops every entry
3. Next 4096 unique lines are ALL cold-start misses
4. Repeat

The renderer calls lineWidth() ~100k times per frame with high temporal locality — the same lines appear across consecutive frames.

Fix

Replace cache.clear() with LRU eviction using plain Map insertion-order semantics:

• On cache hit: delete + set promotes the entry to the end, so it survives future eviction scans
• On cache miss with full cache: evict only the single oldest entry (first key in iteration order), keeping the other 4095 warm

This is a zero-dependency change — no new imports, no new data structures. Map already provides stable insertion order per the ES2015 spec.

Before/After

// Before (sawtooth — catastrophic cache churn)
if (cache.size >= CACHE_LIMIT) {
  cache.clear();  // drops ALL 4096 entries
}

// After (LRU — gradual, keeps cache warm)
if (cache.size >= CACHE_LIMIT) {
  const oldest = cache.keys().next().value;
  if (oldest !== undefined) cache.delete(oldest);  // evict only ONE
}

Verification

• TypeScript: tsc --noEmit -p packages/ink/tsconfig.json passes
• No public API change — lineWidth() signature is unchanged
• Existing rendering behavior is preserved (only the cache hit rate improves)