feat!: migrate from standalone wizer to wasmtime wizer subcommand

[avrabe]

Summary

• Migrate from standalone Wizer toolchain to wasmtime's built-in wizer (v39.0.0+)
• Remove all standalone wizer infrastructure (toolchain, checksums, tools)
• Update wasm_component_wizer rules to use wasmtime_toolchain_type

Breaking Change

Init function name changed: wizer.initialize → wizer-initialize

// Before
#[export_name = "wizer.initialize"]
pub extern "C" fn init() { ... }

// After  
#[export_name = "wizer-initialize"]
pub extern "C" fn init() { ... }

Changes

• Remove toolchains/wizer_toolchain.bzl and related infrastructure
• Remove tools/wizer_initializer Rust tool
• Remove checksums/tools/wizer.json
• Update wasm/wasm_component_wizer.bzl to use wasmtime wizer subcommand
• Update default wasmtime version to 39.0.1
• Update examples to use new wizer-initialize export name
• Update documentation

Test plan

• CI passes (all toolchain and build tests)
• wizer_example builds correctly with wasmtime wizer
• Documentation reflects new wasmtime-based approach

Closes #246