feat(bindings): expose Elements regtest identity

[Abdullah1738]

Summary

• expose a fallible Network::regtest_with_genesis binding constructor;
• bind the caller's exact policy asset and display-order Elements genesis hash;
• document and test the retained standard Elements-regtest parent, address,
  epoch, pegin, and local-client parameters;
• preserve every existing network constructor and generated-binding ABI.

This closes a binding gap for callers that connect to an Elements regtest node
whose authenticated genesis and policy asset differ from LWK's compiled default.
It is intentionally not a general custom-Elements-network constructor.

Validation

• 4 focused network identity/scope tests pass;
• complete lwk_bindings --lib: 43 pass, 0 fail, 1 ignored;
• stable clippy with -D warnings, formatting, and diff checks pass;
• generated C# binding builds and executes with .NET SDK 10.0.100;
• exact identity, uppercase canonicalization, malformed-genesis rejection, and
  legacy binding/new runtime compatibility pass.

The existing no-default-feature binding build remains blocked by the same 11
Lightning-gated UniFFI errors on the unchanged base and this branch.

[Abdullah1738]

The only failing check is the Kotlin framework build. It fails while compiling aws-lc-sys 0.41.0 armv7 Android assembly on the macOS runner (unterminated conditional directives in boringssl_prefix_symbols_asm.h); the Rust/C#/C++/Python-wheel and other binding checks pass, and recent runs on the same base and current master are green. I do not have permission to rerun upstream jobs. Could a maintainer please rerun the failed job?