perf: SIMD scan ASCII runs in input loop

[dyxushuai]

Problem

Input parsing reads one byte at a time and funnels everything through Parser, even when large ASCII runs are present. This adds avoidable per-byte overhead in common input streams.

Fix

Add a SIMD-assisted scan in the input loop to detect contiguous printable ASCII runs (0x20..0x7E). For these runs we emit key_press events directly. If the next byte begins a combining mark, we leave the last ASCII byte for the parser to avoid breaking combining/keycap sequences.

Bench (local, zig build bench, iterations=200, 80x24)

Mixed stream: ASCII + CSI + UTF-8

Baseline ns/frame	SIMD ns/frame	Δ ns/frame	Δ%	Speedup
24,020	5,481	-18,539	-77.2%	4.38x

Improvement: -18,539 ns/frame (-77.2%), 4.38x speedup.

Tests

• zig build test
• zig build bench

[Copilot]

Pull request overview

This PR adds a SIMD-accelerated fast path for scanning and processing contiguous runs of printable ASCII characters (0x20-0x7E) in the input loop, bypassing the parser for these common cases to reduce per-byte overhead. The optimization shows a ~4x improvement in the benchmark (22,725 ns/frame → 5,743 ns/frame for mixed input streams).

Key Changes:

• Added asciiPrintableRunLen() function that uses SIMD vector operations to scan for printable ASCII runs
• Integrated ASCII fast path in the non-Windows input loop to emit key_press events directly for ASCII characters
• Added benchmark functions to compare baseline parser performance against the SIMD-optimized path

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/Loop.zig	Implements asciiPrintableRunLen() SIMD function and integrates ASCII fast path in the input loop before parser invocation
bench/bench.zig	Adds asciiPrintableRunLen() function copy and benchmark harnesses (benchParseStreamBaseline, benchParseStreamSimd) to measure performance improvement

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

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Pull request overview

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Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

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Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

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