fully remove untyped signatures

[darinkishore]

we need to figure out some way to have it be type checked though, which seems tricky, because we want to preserve the behavior of DSPy modules where modules can add (or mutate) fields.

other issues are examples and optimizers.

wondering if metasignature is still needed

[darinkishore]

Codebase audit — remaining untyped signature usage

Where Example (untyped HashMap) still lives:

1. Cache (utils/cache.rs):

   • Cache::get(&self, key: RawExample) — cache key is derived from Example
   • Cache::insert(&mut self, key: RawExample, ...)
   • CacheEntry { prompt: String, prediction: Prediction } — Prediction is also untyped

2. Serialization (data/serialize.rs):

   • load_jsonl() returns Vec<Example> (untyped)
   • save_examples_as_jsonl() takes Vec<Example>

3. Prediction (data/prediction.rs):

   • Prediction likely wraps untyped data (need to check)

4. Macros (lib.rs):

   • field! macro emits serde_json::Value via schemars
   • sign! macro creates Predict::<__InlineSignature>::new() but doesn't wire up typed infra

5. __macro_support re-exports schemars solely for field!

6. Specials (core/specials.rs):

   • History, ToolCall derive schemars::JsonSchema

schemars removal path:

1. Migrate specials.rs to BamlType/Facet (Migrate specials.rs (History, ToolCall, NoTool) to typed architecture #72)
2. Modernize field!/sign! to emit typed SignatureSchema (Modernize field!/sign!/hashmap! macros to emit typed SignatureSchema #73)
3. Remove schemars from __macro_support and Cargo.toml

Example removal path:

1. Cache key → hash of CompletionRequest (Cache: fix key to be request-based, not Example-based #71)
2. CacheEntry → store raw messages + BamlValue, not Prediction
3. load_jsonl/save_examples_as_jsonl → deprecate or make generic over Serialize/Deserialize
4. Then Example can become pub(crate) or removed entirely

MetaSignature: probably not needed once Example is gone. The typed SignatureSchema + Facet reflection covers field mutation (optimizers already use named_parameters walker to find and mutate Predict leaves).