Typechecker improvements: bug fixes, better error messages, dead code removal

[Eugeny48]

This PR improves the Flow9 typechecker in three areas:

Bug fixes:

• Fixed incorrect type inference when if-else branches return different struct types from the same union — the typechecker was widening the result type by merging inner type parameters instead of preserving the concrete branch types. This affected real code like wigi2xliff_light.flow.

Better error messages:

• typeverifyFiFunctionDec and typeverifyFiGlobalVar now report the actual type mismatch (body/value type vs declared type) instead of just saying "in function 'X'" with no details about what went wrong.

Dead code removal:

• Removed the findCommonFUnion function which was unused.

[Eugeny48]

It's ready to merge, just waiting for this PR https://git.area9lyceum.com/Lyceum/lyceum/pulls/10716