Add fallible TBLIS dynamic loading

[shinaoka]

Summary

• add try_dyload_lib() for callers that need to handle an unavailable TBLIS shared library without panicking
• add a structured DyLoadError for library-discovery and required-symbol failures
• keep dyload_lib() as the backward-compatible panic-on-failure wrapper
• leave failed attempts uncached so callers can update loader environment variables and retry

Motivation

Dynamic library absence is a normal runtime condition for applications that treat TBLIS as an optional provider. The current dyload_lib() API reports discovery failures only through panic!, forcing downstream libraries to use catch_unwind for ordinary availability probing. It also makes that probing incompatible with panic = "abort" applications.

This change preserves the existing API while providing a normal Result path.

Validation

• cargo test -p tblis-ffi --no-default-features --features dynamic_loading --lib
• cargo fmt --all -- --check
• cargo clippy --all-targets --features dynamic_loading -- -D warnings
• cargo clippy --all-targets -- -D warnings
• cargo check -p tblis-ffi --no-default-features