feat(stdlib): add fs_*/env_args/format builtins + my-cli argv forwarding

[hyperpolymath]

Summary

Adds six new stdlib builtins to enable My (Solo dialect) for tooling workloads — file I/O, CLI argv, and string formatting — and a small my-cli extension to forward trailing argv to the program.

Motivation

Pre-this-PR, Solo's runtime stdlib was rich for in-memory + terminal-I/O code (71 builtins) but explicitly not tooling-class — IMPLEMENTATION.md lists std::fs, std::io, std::collections::map etc. as Phase-2 roadmap items. That meant scaffold tools / code generators / audit utilities couldn't be written in My; they had to fall back to Python.

This PR ships the smallest set of additions that lets a real Solo tooling program be written end-to-end. Validated by an out-of-tree scaffold/audit tool (scaffold-component.my for the idaptik-port polyrepo work) running clean against six component repos.

What's added

crates/my-lang/src/stdlib.rs

Builtin	Signature	Notes
fs_write_file	(path: String, content: String) -> Unit	Auto-creates parent dirs
fs_read_file	(path: String) -> String
fs_create_dir_all	(path: String) -> Unit	Explicit dir creation
fs_exists	(path: String) -> Bool
env_args	() -> [String]	Reads host-supplied args, falls back to std::env::args().skip(1)
format	(template: String, args: [Any]) -> String	Positional {} substitution; {{ and }} escape

Plus a static PROGRAM_ARGS: OnceLock<Vec<String>> and pub fn set_program_args(args: Vec<String>) so embedders can supply argv to programs without my-cli's clap consuming flags.

All six are stateless fn(Vec<Value>) -> Result<Value, RuntimeError> pointers consistent with existing builtins. Errors route through RuntimeError::Custom(String) for I/O failures; RuntimeError::TypeError for type mismatches.

crates/my-cli/src/main.rs

Run subcommand grows a args: Vec<String> field with trailing_var_arg = true, allow_hyphen_values = true, so:

my run scaffold.my -- --audit /path/to/repo

forwards --audit /path/to/repo to env_args() rather than clap rejecting --audit. The match arm calls my_lang::stdlib::set_program_args(args) before invoking the interpreter.

Test plan

• cargo build --release --bin my -p my-cli clean (3 pre-existing dead-code warnings unchanged)
• Smoke test exercising every new builtin (/tmp/smoke.my):

  hello foo world 42
  ok: fs_exists returned true
  literal { and middle and }
  argv length: 0
  smoke OK
  Result: Int(0)
  

• End-to-end run of an out-of-tree scaffold tool (scaffold-audit.my) against six idaptik-port component repos — all six OK audits, exit 0.
• Unit tests in crates/my-lang/tests/ for each new builtin (success + error paths). Not added in this PR; happy to follow up.

Open design questions

1. Path semantics. fs_write_file auto-creates parent dirs; fs_create_dir_all is exported separately because some scaffolds want explicit empty dirs. Acceptable, or split write into two funcs?
2. Error type. All I/O errors route through RuntimeError::Custom(String). A new RuntimeError::IoError { path, kind, message } variant would be cleaner long-term but would touch every error-handling path; deferred.
3. format placeholder syntax. Positional {} only. No {name} named placeholders, no {0}/{1} indexed. Aligned with Rust's format! minimum surface; can extend later.
4. env_args semantics. Falls back to std::env::args().skip(1) when set_program_args hasn't been called. Skipping arg[0] matches typical CLI ergonomics but could be debated.

Related

• Surfaced issue Type-checker OOM (16-32 GB allocation) on deeply-nested str_concat / format chains #1 (type-checker OOM on nested str_concat/format chains) — discovered during the validation tooling work; not in scope for this PR but worth knowing about.

Why merge this

Six builtins, ~290 LOC of Rust, no breaking changes. Unblocks the entire tooling category; concretely unblocks the idaptik-port polyrepo scaffolding pipeline that produced this work.

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