checker: fix wrapped fn aliases in multiple returns

[medvednikov]

Fixes #27976.

What changed

• compare interface method multiple-return signatures component by component
• reuse the existing strict wrapped function-alias compatibility rules for each component
• add a regression test for an optional function alias inside a multiple return

Root cause

The checker only applied wrapped function-alias compatibility when the complete method return type was a function. A MultiReturn symbol stopped that comparison before it reached the optional function component, so identical signatures were rejected.

Impact

Concrete methods whose multiple-return signatures exactly match an interface now satisfy that interface when a component is an optional function type alias. Existing incompatible wrapped-alias cases remain rejected.

Checks

• VFLAGS='-gc none' ./vnew -gc none -silent vlib/v/compiler_errors_test.v (1604 passed, 1 skipped)
• VFLAGS='-old-compiler -gc none' ./vnew -old-compiler -gc none -silent test vlib/v/tests/interfaces/ (101 passed)
• VFLAGS='-old-compiler -gc none' ./vnew -old-compiler -gc none -silent test vlib/v/ast/ (5 passed)
• VFLAGS='-old-compiler -gc none' ./vnew -old-compiler -gc none -silent test vlib/v/checker/ (3 passed)

A full vlib/v/ run was also started, but the isolated worktree does not contain the ignored bundled thirdparty/tcc GC runtime, so GC-specific tests failed on missing libgc.dylib before the run completed.

[medvednikov]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: a7e0ccb172

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