feat(scan): optimize NEON structural classification with vqtbl

[membphis]

Summary

• replace the NEON structural-character compare loop with a nibble dual-LUT classifier using vqtbl1q_u8
• derive structural, quote, and backslash masks from per-byte tags, while preserving the cheap in-string quote/backslash probe for string-heavy chunks
• add an exhaustive 256-byte LUT classification test to guard against high/low-nibble false positives

Refs #8, #31.

Test Plan

• rustfmt --check src/scan/neon.rs
• cargo test --release
• cargo test --release --no-default-features
• cargo test --release --features test-panic

Local benchmark notes

Single-run Apple ARM64 local medians for quickdecode.parse + access 3 fields, comparing this branch against local origin/main build:

fixture	main	this branch	delta
small	640943 ops/s	776398 ops/s	+21%
medium	161603 ops/s	214408 ops/s	+33%
100k	105042 ops/s	141844 ops/s	+35%
1m	11981 ops/s	16376 ops/s	+37%