Add recursive functions to Strata Core

[joscoh]

Issue #, if available:

Description of changes: This PR adds support for recursive functions in Strata Core, including their definition in concrete syntax, partial evaluation, and an axiom-based SMT encoding.

Main changes:

1. This PR adds an additional attribute to the DDM @[scopeSelf] that acts like @[scope] but also adds the function's name to its body's scope. This is structured and handled very similarly to @[declareDatatype], which involves a similar recursive structure. Main changes are in AST.lean, Format.lean, Core.lean. This adds a @[cases] metadata attribute to indicate the parameter on which case splitting should occur.
2. Adds fields to Func indicating whether a function is recursive and the argument to case-split on. This information is used in Env.lean when the function is added to the environment. Here we also rule out recursive functions that are not yet supported: polymorphic functions, functions with more sophisticated termination measures, and functions without an explicit cases clause.
3. RecursiveAxioms.lean contains the generation of axioms as LExpr for a recursive function. Generating the axiom is fairly simple, since we just create term e.g. forall h t, length (List.cons h t) = length (List.cons h t) and then partially evaluate the right-hand side. Since recursive functions are marked as inlineIfConstr on their case-splitting parameter, this produces the simplified form.

Other changes

1. Adds support in SMT/DDMTransform/Translate for trigger patterns complex enough to handle constructors applied to arguments (e.g. List.cons h t).
2. Changes the evaluation of if-then-else to always reduce within the then and else branches and changes the small-step semantics to match.

Tests

1. RecursiveAxiomTests.lean tests the generation of recursive axioms as Lambda terms.
2. This PR adds a test to SMTEncoderDatatypeTests.lean that demonstrates the SMT encoding of the recursive function axioms.
3. RecursiveFunctionDDMTest.lean tests the DDM parsing for recursive function.
4. RecursiveFunctionErrorTest.lean demonstrates not-yet-supported recursive functions.
5. RecursiveFunctionTests.lean contain a variety of programs with recursive functions, some of which contain VCs which can be solved purely by partial evaluation and others which require SMT-based reasoning.
6. BinaryTreeSize.lean gives a more complex example that demonstrates inductive reasoning over types and functions.

What is not supported (yet): polymorphic and mutually recursive functions, termination checking.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[MikaelMayer]

This PR adds comprehensive support for recursive functions in Strata Core with a well-designed implementation. The changes are structured logically across the DDM layer (adding @[scopeSelf] metadata), the Lambda dialect (axiom generation), and the Core language (grammar, translation, SMT encoding). The test coverage is excellent, including positive tests, error cases, and a complex example (BinaryTreeSize). The documentation clearly explains the new @[scopeSelf] attribute and its purpose.

The implementation correctly handles the key challenges: bringing the function name into scope for recursive calls, generating per-constructor axioms for SMT reasoning, and properly rejecting unsupported cases (polymorphic recursion, missing decreases clauses). The ITE evaluation change to always reduce both branches is a sensible improvement that enables better partial evaluation.

I have a few minor suggestions for clarity and potential improvements, but overall this is high-quality work that's ready to merge after addressing the comments.