Add inline collection declarations for Smithy IDL 2.1

[sugmanue]

Summary

This PR introduces inline collection syntax for Smithy IDL 2.1, allowing list and map shapes to be declared directly in member target positions:

$version: "2.1"

structure MyStructure {
    names: [String]
    tags: {String: String}
    nested: {String: [Integer]}
}

This is syntactic sugar: the assembler generates synthetic shapes (_SyntheticListOfString, _SyntheticMapOfStringToString, etc.) that are shared across structurally equivalent usages.

Changes

Core model (smithy-model)

• Version.java: Refactored to use a bitmask for feature flags. Added INLINE_COLLECTIONS flag for 2.1.
• Trait.java: Added SerializationMode enum (ALL, AST_ONLY, NONE) and serializationMode() method to control where traits are serialized.
• SyntheticShapeTrait: New trait in smithy.synthetic#generated namespace. Uses AST_ONLY serialization, persists in JSON AST but not IDL. Registered via TraitService SPI.
• SyntheticShapeNaming: Utility for generating deterministic synthetic shape names from target types.
• SyntheticShapeValidator: Critical validator that errors if traits are applied to synthetic shapes, and warns if user shapes use the _Synthetic prefix.
• IdlModelLoader: Parses [Target] and {Key: Value} in member target positions. Creates synthetic shapes with deduplication. Version-gated to 2.1+. Nesting capped at 3 levels.
• SmithyIdlModelSerializer: Emits inline syntax for members targeting synthetic shapes. Excludes synthetic shapes from top-level serialization.
• ModelSerializer: Uses serializationMode() instead of isSynthetic() for trait filtering.

Syntax tree (smithy-syntax)

• TreeType.java: Added MEMBER_TARGET, INLINE_LIST_TARGET, INLINE_MAP_TARGET tree types.
• FormatVisitor.java: Formats inline collections as [Target] and {Key: Value}.

Diff tool (smithy-diff)

• AddedShape/RemovedShape: Suppress events for synthetic shapes.
• ModifiedTrait: Added SyntheticShapeTrait to IGNORED_TRAITS.

CLI (smithy-cli)

• MigrateCommand: Updated to emit version "2.1" and skip any 2.x files.

Documentation

• spec/idl.rst: ABNF grammar update, new "Inline collection declarations" section, version statement note, examples in List/Map/Structure/Union sections.
• spec/aggregate-types.rst: Inline syntax notes in List and Map sections.
• spec/model.rst: Member shapes note, Shape ID _Synthetic prefix reservation, trait restriction note.
• spec/json-ast.rst: New "Synthetic shapes in the JSON AST" section.
• spec/type-refinement-traits.rst: Note that @sparse cannot apply to synthetics.
• spec/selectors.rst: Example for selecting synthetic shapes.
• guides/style-guide.rst: Recommendation to prefer inline for single-use collections.
• guides/evolving-models.rst: Migration guidance for inline vs explicit.
• guides/building-codegen/mapping-shapes-to-languages.rst: Synthetic shape naming guidance.
• guides/building-codegen/using-the-semantic-model.rst: ShapeVisitor note.
• guides/model-validation-examples.rst: Selector note for excluding synthetics.
• guides/smithy-build-json.rst: Tip for selecting synthetics in projections.
• quickstart.rst: Forward-looking tip about inline syntax.
• designs/inline-collections.md: Design document.

Testing

• Valid model tests: Basic inline collections, grouping (shared shapes), union members, use-statement resolution, nested collections.
• Error tests: Version gating (2.0 rejects inline syntax), nesting depth limit (>3 levels).
• IDL round-trip test: Parse, serialize, compare matches original.
• JSON AST round-trip test: IDL to JSON to model comparison.
• Diff tests: Add/remove inline collections, inline to explicit migration, explicit to inline migration, explicit with constraint traits, no-change scenario.

Limitations

• Inline collections cannot carry shape-level traits (@sparse, @uniqueItems, @length on the collection). Use explicit shape definitions for these.
• apply statements cannot target synthetic shapes.
• Nesting is limited to 3 levels.