Add get_erased helper to reduce get/touch monomorphization

[fasterthanlime]

Summary

Follow-up to PRs #47 and #48, addressing more monomorphization opportunities identified in #49.

• Add get_erased() to DerivedCore that returns BoxFuture<ArcAny>, moving the async block out of the generic DerivedIngredient::get method
• Update DerivedIngredient::get to use get_erased, reducing the async state machine size in the generic wrapper
• Update DerivedIngredient::touch to use the existing touch_erased helper

Expected savings

Based on the analysis in #49:

• get::{{closure}}: 21k × 50 copies → much smaller wrapper + one copy per DB type

The remaining high-impact items are:

• TypedCompute::compute::{{closure}} (14k × 60) - This is unavoidable as it calls the user's compute function with concrete types
• Persistence callbacks using facet_postcard - Would need reflection-based serialization

Test plan

• All 66 tests pass
• No clippy warnings

Part of #49

[Copilot]

Pull request overview

Reduces code bloat from monomorphization in the derived ingredient query path by moving more async state-machine logic into type-erased helpers on DerivedCore.

Changes:

• Added DerivedCore::get_erased() returning BoxFuture<PicanteResult<ArcAny>> to avoid a larger generic async block in DerivedIngredient::get.
• Updated DerivedIngredient::get to delegate to get_erased and downcast at the boundary.
• Updated DerivedIngredient::touch to delegate to the existing touch_erased helper.

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