feat: model class-body index signatures (#123)

[nickna]

Part of #80. Implements #123.

class A { [k: string]: T } parsed since #121, but its value type was discarded. This models the index type on the class and consults it when indexing a class instance — mirroring index signatures on interfaces/object types.

Changes

• Parser: TryParseClassIndexSignature returns the parsed Stmt.IndexSignature (key name, key-type token, value type) instead of discarding; ClassDeclaration collects them into a new Stmt.Class.IndexSignatures slot.
• Types: ClassMetadataCore, MutableClass, and TypeInfo.Class gain String/Number/SymbolIndexType (optional defaulted — existing construction sites unaffected). CheckClassDeclaration / CheckDeclareClass resolve the signatures inside the class type-environment scope (so value types can reference the class's own type parameters).
• Index access: indexing a class instance (TypeInfo.Class or Instance<Class>) by string/number/symbol returns the matching index value type, including the TS number-key-uses-string-index fallback. Excess-property checking treats a class index signature like an interface/record one.

Verification

• New ClassIndexSignatureTests (7): parsing, string/number index access, type-mismatch errors, numeric-key fallback, ambient declare class, and computed-property-name disambiguation.
• Full xUnit suite green except the 2 known pre-existing packaging failures.
• TS conformance baseline unchanged — zero regressions.

Scope notes

• Index-signature value-type assignability (e.g. a number-index not assignable to a string-index target) is intentionally not implemented here — it's a pre-existing general gap (not enforced for Record↔Record either) and orthogonal to this feature.
• As with TS Conformance: model call/construct signatures on object type literals (not just interfaces) #122, the conformance baseline can't reflect these negative assignmentCompatWith{String,Numeric}Indexer tests until TS Conformance: propagate TSnnnn codes through the recovery path (unlocks negative-test matching) #125 (recovery-path TsCode) lands.
• Class expression index signatures (const C = class { [k]: T }) are deferred (rare); only class declarations are covered.