perf: improve terminal frame pacing and diff rendering

[KoalaHao]

Summary

• Batch adjacent changed terminal cells during differential rendering to reduce cursor moves and write overhead.
• Share scheduler frame pacing logic between bindings.
• Track the next target frame time independently from late timer callbacks, so small overshoots do not permanently lower effective FPS.
• Add focused regression coverage for contiguous diff writes, wide glyphs, batching, and frame pacing drift.

Tests

• dart analyze
• dart test test/rendering/terminal_diff_output_test.dart test/foundation/scheduler_batching_test.dart