Fix cross-module generic type alias export and add coverage

[ASDAlexander77]

Summary

• Last remaining item from decls-cross-module-declaration-mechanism's unverified list (after generic classes/Fix cross-module generic class export/import (4 co-operating bugs) #280, generic functions/Fix cross-module generic function export and add coverage #285, generic interfaces/Fix cross-module generic interface export and add coverage #286): genericTypeAliasMap's registration path in mlirGen(TypeAliasDeclaration) never called any export function for the generic branch - only the non-generic else-branch calls addTypeDeclarationToExport. Confirmed with a cross-module repro: a generic type alias failed cross-module resolution under -shared.
• Unlike the other three generic declaration kinds, there is no dedicated GenericTypeAliasInfo struct tracking sourceFile/elementNamespace for a generic type alias - genericTypeAliasMap's value is just {typeParams, typeNode}, since resolution (getTypeByTypeReference / resolveGenericTypeInNamespace) is pure compile-time type substitution on the stored TypeNode, never re-invoking mlirGen on the whole declaration the way a class/function/interface instantiation does. So instead of adding a new Info struct, addGenericTypeAliasDeclarationToExport takes the AST node and namespace directly and is called inline from the one registration site, gated on stage == Stages::SourceGeneration (a given top-level TypeAliasDeclaration is visited exactly once per stage, so this is sufficient dedup - no "already registered" early-return branch exists here to retry from, unlike the class/function/interface fixes).
• Tested with both a single- and two-type-param alias (Box<T>, Pair<A,B>) to check for the classes/functions' export-bleed-on-instantiation bug; neither triggered it, matching generic interfaces' outcome - a type alias has no compiled body or runtime entity at all, so there's nothing for an instantiation to wrongly dllexport in the first place. All 3 tiers passed on the first attempt - the simplest of the four generic-declaration-kind fixes.
• This closes out the generic cross-module export arc: classes (4 bugs) → functions (2 bugs) → interfaces (1 bug) → type aliases (1 bug, first try).

Test plan

• New export_type_alias_generic.ts / import_type_alias_generic.ts cross-module pair (single- and two-type-param generic type aliases), 3 tiers: test-compile-export-import-type-alias-generic, test-compile-shared-export-import-type-alias-generic, test-jit-shared-export-import-type-alias-generic
• Full suite: ctest -C Debug -j8 — 802/802 green (799 previously green + 3 new)

🤖 Generated with Claude Code

[ASDAlexander77]

Rebased/merged origin/main to resolve the conflict (commit f17792d) - full suite re-verified at 805/805 green.