Fix distinct borrow handling for bracket operators and restrict type-level borrow pragma to .

[Clemente90]

Motivation

• Recent changes allowed [] in the distinct type-level {.borrow.} pragma and introduced regressions in bracket-operator borrowing for builtin-like containers; the language manual limits type-level borrow to dot only and bracket operators must be borrowed via procs.
• The compiler must correctly resolve proc-level {.borrow.} for magic bracket ops (array/seq/openArray/etc.) across multiple distinct types, generic instantiations and block scopes without creating global ambiguities.

Description

• Restrict type-level borrow pragma to accept only . and emit a clear error for attempts to use [] in type declarations by updating compiler/pragmas.nim.
• Stop treating borrowed bracket operators as a type-level flag by removing/controlling tfBorrowBrackets propagation and ensure []. borrows are handled via proc-level logic in compiler/semstmts.nim, compiler/semexprs.nim and compiler/semcall.nim.
• Make overload/borrow resolution robust for magic bracket operators by skipping return-type checks for bracket-named borrows and preventing the automatic magic-match path from producing ambiguities for borrowed procs via changes in compiler/semcall.nim and compiler/sigmatch.nim.
• Preserve correct handling for generic/instantiated distinct parameter types when marking borrowed [] proc parameters, including tyGenericInvocation cases, via compiler/semstmts.nim adjustments.
• Update AST and encoding helpers so flags remain consistent (compiler/astdef.nim, compiler/ic/enum2nif.nim) and rework the semantic subscript handling to respect borrow flags in compiler/semexprs.nim.
• Tests: remove the now-invalid type-pragma test tests/distinct/tborrow_type_pragma_brackets.nim, add tests/distinct/tborrow_seq_brackets.nim, and update several tests/distinct/* files to use proc-level {.borrow.} for brackets and exercise multiple distinct types, generics and block-scope cases.

Testing

• Rebuilt the compiler and tools with ./bin/nim c ... koch and ./koch boot and the build succeeded.
• Ran modified/added test cases individually with the new compiler: ./bin/nim r tests/distinct/tborrow_array_brackets.nim, ./bin/nim r tests/distinct/tborrow_negative_generics_and_blockscope.nim, and ./bin/nim r tests/distinct/tborrow_seq_brackets.nim, all of which executed successfully.
• Ran the invalid/negative check ./bin/nim check --hints:off --warnings:off tests/distinct/tinvalidborrow.nim and it produced the expected reject diagnostics.
• Overall: updated tests now pass and the reject-case emits the expected error messages.

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Codex Task