Let weaveffi new scaffold the starter IDL in JSON or TOML

[owenthcarey]

Summary

WeaveFFI accepts IDLs in YAML, JSON, and TOML everywhere (weaveffi generate, validate, lint, and so on pick the parser from the file extension), but weaveffi new <name> always scaffolds weaveffi.yml. Users who keep their configs in JSON or TOML have to hand-convert the starter file.

Proposed change

Add an --idl-format <yaml|json|toml> flag (default yaml) to the New subcommand in crates/weaveffi-cli/src/main.rs and thread it into cmd_new in crates/weaveffi-cli/src/commands/new.rs. Rather than maintaining three hand-written string templates, the simplest robust approach is to parse the existing YAML starter with weaveffi_ir::parse::parse_api_str and re-serialize it in the requested format (the format command already does canonical serialization per format, see crates/weaveffi-cli/src/commands/format.rs for the pattern). Name the output file weaveffi.yml, weaveffi.json, or weaveffi.toml accordingly, and update the generated README and any printed next-step hints to reference the right filename.

Acceptance criteria

• weaveffi new demo --idl-format toml produces a weaveffi.toml that passes weaveffi validate
• Same for json; plain weaveffi new demo still produces the YAML file byte-identical to today's output
• An invalid format value is rejected at argument parsing (use a clap ValueEnum)
• Tests cover the three formats (see the existing CLI tests in crates/weaveffi-cli/src/main.rs or the tests in commands/new.rs if present)

Pointers

• Subcommand definition: Commands::New in crates/weaveffi-cli/src/main.rs
• Scaffolding logic and the starter IDL template: crates/weaveffi-cli/src/commands/new.rs
• Canonical serialization per format: crates/weaveffi-cli/src/commands/format.rs

Suggested commit message

feat: let weaveffi new scaffold the starter IDL in JSON or TOML