Migrate from JSON to YAML configuration format

[akesson]

Summary

This PR migrates the builder tool from JSON to YAML configuration format for better readability and a more standard configuration file format.

Breaking Changes

• Configuration files must now be in YAML format - The builder binary now only accepts .yaml/.yml files, not JSON
• Default config filename changed from builder.toml to builder.yaml

Key Changes

YAML Migration

• Added serde_yaml to workspace dependencies
• Updated builder binary to parse YAML configuration files (removed JSON conversion logic)
• Modified BuilderCmd::exec() and run() methods to write YAML instead of JSON
• Changed default config filename from builder.toml to builder.yaml
• Updated documentation references from JSON to YAML

Build System Fixes

• Fixed cargo deadlock in crates/examples/build.rs by removing nested cargo build -r -p builder call
• Build script now looks for existing builder binary instead of building it
• Generates stub assets.rs when builder binary doesn't exist, allowing clean workspace builds
• Prevents infinite hang during cargo build --release

Code Quality

• Fixed clippy warnings: needless_borrows_for_generic_args, len_zero
• Ran cargo fmt for consistency
• All 87 tests pass

Bug Fixes

• Create parent directories when writing debug symbols (wasm crate)
• Create parent directories when writing site files (site_fs)
• Fix manifest_path to point to Cargo.toml instead of directory (swift_package)

Test Plan

• ✅ Clean build from scratch works: cargo clean && cargo build
• ✅ Release build works without hanging: cargo build --release
• ✅ All 87 tests pass: cargo nextest run
• ✅ Clippy passes with -D warnings
• ✅ YAML serialization/deserialization works correctly with tagged enums

Example YAML Configuration

log_level: Normal
log_destination: Terminal
release: false
builder_toml: ./builder.yaml
in_cargo: true
cmds:
- !Copy
  src_dir: assets
  recursive: true
  file_extensions: []
  output:
  - dir: dist
    brotli: true
    checksum: true
- !Wasm
  package: my-wasm-package
  profile: Release

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