fp-hw-survey: add cross-hardware FP behavior survey crate (capture/me…

[EmJayGee]

This pull request introduces a new Rust crate, fp-hw-survey, to the workspace. The crate is a dependency-free tool for surveying native floating-point hardware behavior across different architectures, capturing and merging results to identify hardware divergences. The main changes include adding the crate to the workspace, providing documentation, setting up architecture-specific backends, and handling unsupported architectures.

New crate addition and documentation:

• Added the fp-hw-survey crate to the workspace, with a new entry in the root Cargo.toml and its own Cargo.toml specifying a dependency-free, cross-platform Rust binary for floating-point hardware surveys. [1] [2]
• Created a comprehensive README.md in crates/fp-hw-survey/ detailing the tool's purpose, usage, supported architectures, record formats, and contribution process.
• Added a .gitignore for the crate to exclude build artifacts, NDJSON files, and scratch directories.

Architecture backend setup:

• Implemented modular architecture dispatch in src/arch/mod.rs, selecting the appropriate backend (aarch64, x86_64, or a fallback) at compile time and exposing a uniform interface for the rest of the tool.
• Added an unsupported backend in src/arch/unsupported.rs that disables capture functionality on architectures without hand-written assembly, ensuring graceful fallback.