feat: add no_std + alloc support

[lilith]

Adds no_std + alloc support for embedded/WASM use cases.

Closes #2

Changes

• Add #![cfg_attr(not(feature = "std"), no_std)] for conditional no_std
• Use hashbrown::HashMap in no_std mode (std uses std::collections::HashMap)
• Add num-traits with libm feature for f32 math methods in no_std
• Replace std::arch → core::arch for SIMD intrinsics
• Replace std::cmp, std::fmt, std::hash, std::mem, std::slice → core::*
• Use core::error::Error (stable since Rust 1.81)
• Feature-gate num_traits::Float import for no_std only
• Fix pre-existing collapsible_if clippy warnings in vpsearch.rs

Usage

# Default (std) - unchanged
quantizr = "1.4.3"

# no_std + alloc  
quantizr = { version = "1.4.3", default-features = false }

Testing

• Builds on thumbv7em-none-eabihf (no_std target)
• All existing tests pass
• Clippy clean with -D warnings

[DarthSim]

Hey @lilith!

I'm up to add no_std support, yet I'm not ready to test it right now, as I'm fully loaded at the moment.

Here are some comments I can make after the first glance:

1. The dependencies needed for the no_std mode should be optional. Not sure how to do this without an additional no_std feature though.
2. I'd add a tiny module named something like no_std_compat that just includes things needed for no_std: alloc::vec::Vec, alloc::boxed::Box, num_traits::Float, etc. I'd include this module in every file:

#[cfg(not(feature = "std"))]
use crate::no_std_compat::*

Also, it's probably a good idea to use hashbrown::HashMap even in std mode. It should be faster than std::collections::HashMap, and we don't need cryptographic security in the histogram. But I need to benchmark it first.