[codex] Facet runtime API cleanup and adapter boundary hardening

[darinkishore]

Summary

This PR performs a one-shot facet-only compatibility sweep and then hardens the long-term public API boundary based on an adversarial audit.

Primary changes:

• Removes bamltype::compat usage from typed/runtime/macro paths.
• Promotes schema registration internals into a dedicated core module and runtime traits module.
• Renames adapter trait from BamlAdapter to FieldCodec.
• Removes the public with_constraints helper and inlines constraint attachment in generated macro output.
• Adds an explicit adapter extension namespace (bamltype::adapters) and moves adapter-oriented API there.
• Keeps core runtime surface focused (BamlType*, conversion, rendering/parsing), with adapter hooks intentionally namespaced out of the top-level API.

Why

The repository is early-stage and not constrained by strict back-compat requirements, so this is a deliberate cleanup to avoid carrying bridge-era artifacts forward.

The adversarial audit found that leaving registry+adapter hooks as root-level core APIs would create long-term semver lock-in around internals likely to evolve. This PR incorporates the follow-up hardening to avoid that lock-in.

Detailed changes

bamltype core

• Added crates/bamltype/src/runtime.rs for runtime traits/error types.
• Renamed/moved registry implementation into crates/bamltype/src/schema_registry.rs (SchemaRegistry) and wired schema builder to it.
• Added crates/bamltype/src/adapters.rs:
  • adapters::FieldCodec
  • adapters::AdapterSchemaRegistry
• Removed crates/bamltype/src/compat.rs.
• Updated facet_ext, convert, and schema_builder to new module boundaries.

macro integration

• crates/dsrs-macros/src/lib.rs:
  • Stops generating bamltype::compat::* paths.
  • Inlines TypeIR constraint extension instead of using with_constraints.
  • Drops generated BamlTypeInternal::register(...) delegation.
• crates/bamltype-derive/src/lib.rs:
  • Uses runtime_crate::adapters::FieldCodec in generated adapter hooks.

dspy-rs integration

• Removed compat-path usage in typed interfaces and runtime code.
• Updated root exports to canonical bamltype symbols only.

tests

• Updated all adapter/registry path usage to bamltype::adapters::* (and corresponding facet_runtime::adapters::* paths).
• Updated compat-path imports in macro and dspy tests.

Validation

• cargo fmt --all
• cargo check --workspace
• cargo test -p bamltype --tests
• cargo test -p dsrs_macros --tests
• cargo test -p dspy-rs --tests
• cargo test --workspace

All passed.

User impact

This is intentionally breaking for old compat paths and root-level adapter/registry imports. The result is a cleaner facet-native API surface and a clearer extension boundary for custom field codecs.