Refactor: Split sml_core (DOM+Parser) and sml_schema (Schema)

[berkeleynerd]

Summary

• Split this repo into two crates to reduce dependency surface and speed builds:
  • sml_core: SML, SML.DOM, SML.DOM.Parser, SML.DOM.Builder, SML.DOM.Writer, SML.UTF8, SML.Entities, SML.Memory_Model.
  • sml_schema: SML.Schema only; depends on sml_core.

Motivation

• Consumers that only need parsing/DOM should not pull schema.
• Faster proofs/builds for core‑only changes; clearer ownership and release cadence.

Out of Scope (for this issue)

• Public API changes beyond moving files.
• New features or behavior changes.

Proposed Plan (high‑level)

1. Create new repo/crate: sml_core (this repo renamed or copied), remove sml-schema*.ad[sb] and schema tests.
2. Create new repo/crate: sml_schema, add schema sources and tests; set depends = ["sml_core ^0.1"] in alire.toml.
3. Update examples to depend on sml_schema (which pulls sml_core).
4. CI: add alr test in each crate; verify cross‑build on macOS/Linux.

Acceptance Criteria

• alr build and alr test pass in both crates.
• sml_core exports the same Library_Interface minus schema; sml_schema exports only SML.Schema.
• No cyclic dependencies; compile without warnings elevated to errors in tests.

Risks / Mitigations

• Version skew: start with lockstep minor versions; add CI matrix building sml_schema against latest sml_core.
• API boundaries: keep package specs unchanged; only relocate files.

Notes

• Alternative is a single‑crate conditional build (GPR external SML_WITH_SCHEMA), but separate crates provide cleaner dependency graphs.