Fix/wasix subprocess feature detection

[Arshia001]

This PR allows WASIX subprocesses to select a different engine or different feature set relative to the parent process.

Fixes #6515.

[Copilot]

Pull request overview

This PR improves WASIX subprocess execution by allowing child modules to be compiled with a different engine backend and/or an extended Wasm feature set than the parent process (addressing macOS failures where subprocess modules require features like exceptions).

Changes:

• Add runtime-side module feature detection and sys-backend selection/caching so subprocesses can compile with a compatible backend/features.
• Update PluggableRuntime construction to take an Engine up-front (removing post-construction engine mutation patterns) and adjust call sites.
• Add a new WASIX test that runs a --singlepass parent which execs an exception-using child module to validate engine/feature switching.

Reviewed changes

Copilot reviewed 26 out of 28 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/wasix/subprocess-engine-selection/run.sh	Builds parent/child WASIX modules with different feature requirements and runs the parent under Singlepass.
tests/wasix/subprocess-engine-selection/main.c	Parent module vfork/execs the child and validates its exit code.
tests/wasix/subprocess-engine-selection/child.cc	Child module uses C++ exceptions to require exception-handling support.
tests/wasix/subprocess-engine-selection/.no-build	Marks the test as manually built via run.sh.
tests/lib/wast/src/wasi_wast.rs	Updates PluggableRuntime construction to pass the current store engine.
lib/wasix/tests/wasm_tests/reflection_tests/wasix-reflection-and-closures/main.c	Attempts to make closure/reflection test more conditional, but changes affect compilation.
lib/wasix/tests/wasm_tests/.gitignore	Normalizes an ignore entry formatting (no functional change).
lib/wasix/tests/runners.rs	Updates runtime construction to pass an engine argument.
lib/wasix/src/syscalls/wasix/proc_exec3.rs	Removes an unused new_store local related to exec/spawn flow.
lib/wasix/src/state/builder.rs	Constructs PluggableRuntime with an engine derived from the builder config.
lib/wasix/src/runtime/mod.rs	Core change: add sys-backend selection, engine caching, and per-module feature detection in resolve_module; update PluggableRuntime::new signature.
lib/wasix/src/runners/wasi.rs	Updates tests to the new PluggableRuntime::new(..., engine) signature.
lib/wasix/src/os/console/mod.rs	Updates tests to the new PluggableRuntime::new(..., engine) signature.
lib/wasix/src/bin_factory/binary_package.rs	Updates tests to the new PluggableRuntime::new(..., engine) signature.
lib/wasix/Cargo.toml	Adds sys/compiler backend feature wiring (sys-default, singlepass, cranelift, llvm) and an optional compiler dependency.
lib/swift/src/lib.rs	Updates runtime construction to pass an engine.
lib/compiler/src/engine/inner.rs	Adds Engine::new_with_extended_features(...) helper for sys engine rebuilding.
lib/compiler/src/compiler.rs	Extends the Compiler trait with a configuration() accessor.
lib/compiler-singlepass/src/compiler.rs	Implements new Compiler::configuration() API.
lib/compiler-llvm/src/compiler.rs	Implements new Compiler::configuration() API.
lib/compiler-cranelift/src/compiler.rs	Implements new Compiler::configuration() API.
lib/cli/src/commands/run/wasi.rs	Passes the selected engine into PluggableRuntime::new and removes set_engine chaining.
lib/cli/Cargo.toml	Ensures CLI backend feature flags also enable corresponding wasmer-wasix/* backend features.
lib/c-api/src/wasm_c_api/wasi/mod.rs	Passes the store engine into PluggableRuntime::new.
lib/c-api/Cargo.toml	Ensures C-API backend feature flags also enable corresponding wasmer-wasix/* backend features.
examples/wasi.rs	Updates example to construct PluggableRuntime with Engine::default().
Cargo.toml	Ensures workspace backend features also enable corresponding wasmer-wasix/* backend features.
Cargo.lock	Records the added wasmer-compiler dependency.

[marxin]

I thought it's already something we've been doing, see #6222.