Add getIonDeserializer% and getIonSerializer% elaborators for generic Ion serialization

[keyboardDrummer-bot]

Closes #5

Summary

Adds two term-level elaborators for generic Ion serialization/deserialization of Lean types:

• getIonDeserializer% — generates a function ByteArray → Except Std.Format α that deserializes Ion binary data into values of a given Lean type at compile time.
• getIonSerializer% — generates Java source files (records, sealed interfaces) with toIon methods that serialize to the same Ion format.

Design

Since Type is erased at runtime in Lean, both elaborators inspect the type's constructors and fields in the Lean environment at elaboration time and produce syntax that is then type-checked normally.

Ion encoding conventions

Lean type	Ion encoding
Structures	Ion struct with field names as keys
Single-constructor inductives	Ion struct with positional keys _0, _1, …
Multi-constructor inductives	Ion sexp (ConstructorName arg₁ arg₂ …)

Supported leaf types

Nat, Int, Float, String, Bool, Decimal

Container types

List α → Ion list / java.util.List<T>, Option α → Ion null for none / nullable T

Nested and recursive types

Fields whose types are themselves structures or inductives are handled recursively. For recursive types (e.g., Tree), the generated code uses let rec bindings — the enclosing definition must be marked partial.

Usage

-- Deserializer
def deserializePoint : ByteArray → Except Std.Format Point :=
  getIonDeserializer% Point

-- Java serializer
def pointJava := getIonSerializer% Point "com.example"

Files

• Strata/Util/IonDeserializer.lean — Runtime helpers and the getIonDeserializer% elaborator
• Strata/DDM/Integration/Java/Gen.lean — Rewritten to provide getIonSerializer% for Lean types
• Strata/DDM/Integration/Java/GenDDM.lean — Original DDM-based Java generator (preserved for javaGen CLI)
• StrataTest/Util/IonDeserializer.lean — Tests for the deserializer
• StrataTestExtra/DDM/Integration/Java/TestGen.lean — Rewritten tests for the serializer

Testing

• All existing and new tests pass (583 test jobs).
• Java compilation and roundtrip tests verify that generated Java code compiles with javac and produces Ion data that getIonDeserializer% can read back correctly.
• Validated for JVerify compatibility: Switch Laurel AST to getIonSerializer% format jverify#405