perf(editor): honor kSyntaxHighlightIsolateThreshold

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

kSyntaxHighlightIsolateThreshold = 8192 is defined but never consulted. Every debounced edit sends full code + grammarJson + theme JSON through compute, so small buffers pay isolate overhead and large buffers resend grammar/theme repeatedly.

Evidence

• lib/core/editor/syntax_highlight_isolate.dart (const at top; highlightOffMainThread always compute)
• lib/core/editor/querya_highlight_controller.dart

Acceptance

• Buffers below threshold highlight inline (or cheaper path)
• Above threshold uses isolate; grammar/theme cached on worker when possible
• No typing jank regression on small SQL editors

Suggested fix

Branch on threshold; cache highlighter/theme on the worker isolate; avoid resending grammar every keystroke.

[ZhuchkaTriplesix]

Shipped on dev via #436 (Closes did not auto-close because PRs targeted dev).