perf: add ASCII fast path in parseGround

[dyxushuai]

Problem

parseGround handles the common ASCII keypress path by running UTF-8 decoding and grapheme iteration even for single-byte printable ASCII. This adds avoidable overhead for each keystroke.

Fix

Add a fast path for printable ASCII (0x20..0x7E) that returns a keypress directly. We only take the fast path when the next byte is ASCII (or absent) to avoid multi-codepoint graphemes like combining marks/keycap sequences that start with ASCII but continue with non-ASCII bytes. Control bytes and ESC keep the existing mapping rules.

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

Baseline = pre-fast-path work (UTF-8 + grapheme iteration)
Fast = current parseGround fast path

Case	Baseline ns/frame	Fast ns/frame	Improvement	Speedup
ASCII 'a'	194	51	-73.7%	3.80x
Mixed stream (ASCII + CSI + UTF-8)	5,465	3,464	-36.6%	1.58x

Improvement: ASCII 'a' -73.7% (3.80x); Mixed stream -36.6% (1.58x).

Tests

• zig build test
• zig build bench

[Copilot]

Pull request overview

This PR adds a performance optimization for parsing ASCII keypress events by introducing a fast path that bypasses UTF-8 decoding and grapheme iteration for printable ASCII characters (0x20-0x7E). The fast path only applies when the next byte is also ASCII or absent, ensuring multi-codepoint graphemes like combining marks and keycap sequences that start with ASCII are correctly handled by the slower path.

Key changes:

• Added fast path in parseGround for printable ASCII with safety check for multi-byte sequences
• Added benchmark comparison showing 3.35x speedup for ASCII input
• Added uucode dependency to benchmark imports for baseline measurement

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Parser.zig	Adds fast path for ASCII characters (0x20-0x7E) that skips UTF-8/grapheme processing when next byte is ASCII or absent
bench/bench.zig	Adds baseline benchmark function and comparison tests for ASCII parsing performance
build.zig	Adds uucode module import to benchmark executable for baseline comparison

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