Improve distinct borrow matching and add tests

[Clemente90]

Motivation

• Borrow resolution produced incorrect matches, crashes and unintended instantiations for nested distinct and generic types; normalizing/peeling those wrappers and avoiding instantiation during borrow lookup is required.

Description

• Introduced guarded normalization and unwrapping helpers (normalizeBorrowType, baseTypeFromDistinctGeneric, borrowSearchType) and a scoring matcher borrowMatchScore to preference best overloads when resolving borrows, and integrated them into searchForBorrowProc (compiler/semcall.nim).
• Avoided recursive/erroneous instantiation during borrow resolution by adding the inBorrowSearch context guard and using borrowInstType to compare types without triggering generic instantiation (compiler/seminst.nim, compiler/semdata.nim).
• Adjusted distinct-base collapsing and generic-instantiation handling so nested distinct/generic wrappers are collapsed safely when computing borrow compatibility (compiler/seminst.nim, compiler/semcall.nim).
• Added focused tests exercising nested distinct/generic borrow edge cases: tests/distinct/tborrow_nested_distinct_len.nim, tborrow_static_array_nested.nim, tborrow_ref_object.nim, and tborrow_string_concat.nim.

Testing

• Built the compiler and ran focused compiles of the new tests with bin/nim c -d:release -r compiler/nim c -d:release -r tests/distinct/tborrow_nested_distinct_len.nim, .../tborrow_static_array_nested.nim, .../tborrow_ref_object.nim, and .../tborrow_string_concat.nim, all of which completed successfully.
• Ran the distinct test suite with bin/testament --nim:compiler/nim c tests/distinct, which reported 24 passed and 1 failure; the lone failure is a JS-backend invocation failing because the compiler binary in this environment was not built with the JS backend, not a regression introduced by these changes.

---

Codex Task