Exported generators under -shared: print boxed objects and quote internal field names

[ASDAlexander77]

Closes #314.

Problem

A generator exported from a shared library returned garbage (or the importer failed to compile) when used from a program built with -shared. Generator methods on exported classes (*items()) failed the same way. Static multi-file builds were not affected because they don't go through the embedded declaration text (__decls).

Cause

Two bugs in the declaration text a library embeds for importers:

1. Boxed object printed as a value. A generator returns its object by reference (ObjectType over a tuple), but it was printed as a plain {...}, which the importer reads back as a value tuple. Library and importer disagreed on the layout.
2. Internal field name printed unquoted. The generator object's .step field was printed as .step:s32, which is a syntax error. The importer dropped the whole declaration, and compilation then failed in for...of with "Element access with a non-constant index".

Fix

• New built-in generic type BoxedObject<T> (MLIRGenTypes.cpp) that resolves to ObjectType over T. The declaration printer now writes objects held by reference as BoxedObject<{...}>.
  • Named BoxedObject rather than Boxed: user generic types resolve before built-in ones, and an existing test already declares class Boxed.
• Field names that aren't identifiers are printed quoted (".step": s32), which parses back to the exact name.
• Both are opt-in flags on MLIRPrinter (printBoxedObjectTypes, quoteNonIdentifierFieldNames), set only by MLIRDeclarationPrinter, so type printing in diagnostics is unchanged.
• A top-level @boxed variable still prints its bare storage shape; @boxed already adds the extra dereference, so BoxedObject<> there would dereference twice.

Declaration text before and after:

function functionGenerator() : {.step:s32, next(): {value:s32, done:boolean}};
function functionGenerator() : BoxedObject<{".step":s32, next(): {value:s32, done:boolean}}>;

Tests

• export_generators.ts / import_generators.ts: function*, const generator, class generator method, namespaced generator, direct next() calls. Registered as static, -shared and -jit -shared tests; all pass.
• Unit tests in unittests/MLIRGen/DeclarationPrinter.cpp: boxed return type, bare shape for @boxed variables, quoted non-identifier field names.
• Full release suite (Windows): 2716/2716 passed.

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