Fix stubbing bugs: FFI name resolution and generic parameter validation

[feliperodri]

Fixes three stubbing bugs that block stabilization of the stubbing feature.

Resolves #1953
Resolves #2686
Resolves #2673
Partial progress on #2007

Bug 1: Foreign function stubs fail name resolution (#2686, #2673)

Functions declared in extern "C" { fn foo(); } blocks could not be stubbed because the name resolution algorithm (resolve_relative) only searched top-level module items. Foreign functions are children of ForeignMod HIR items, not top-level items themselves.

Additionally, the stub validation in attributes.rs rejected foreign functions with "function does not have a body, but is not an extern function" — the check was intended to catch trait functions without default bodies, but it also caught extern "C" functions.

Fix:

• resolve.rs: When a name is not found as a direct module item, also search inside ForeignMod blocks by iterating their HIR foreign items.
• attributes.rs: Exclude is_foreign_item from the no-body rejection.

Bug 2: Generic parameter names must match (#1953)

Stub validation compared MIR body types directly, which meant bar<S>() -> S was rejected as a stub for foo<T>() -> T because the generic parameter S produced a different Ty than T. The RFC explicitly states this should work.

Fix:

• stubbing/mod.rs: Build a substitution mapping the stub's generic parameters to the original's parameters by position, using GenericArgs::identity_for_item as the base to correctly handle parent type parameters (e.g., methods on LocalType<T>). Apply the substitution to the stub's types before comparing.

Bug 3: Lifetime mismatch detection (partial, #2007)

Type comparison now uses MIR body types (which have regions erased) combined with the generic renaming substitution. This means lifetime differences between the original and stub (e.g., &'a self vs &char in method-to-function stubs) no longer cause false rejections. A documentation note warns users that lifetime mismatches can still cause subtle verification failures.

Tests

New tests:

• stub_foreign_function.rs — basic extern "C" function stubbing
• stub_multiple_foreign_functions.rs — multiple foreign functions from one extern block
• stub_generic_param_rename.rs — single generic param T → S
• stub_generic_multi_param_rename.rs — multiple generic params A, B → X, Y

Promoted from fixme:

• fixme_issue_1953.rs → stub_generic_param_rename_simple.rs (was ignored, now passes)

Documentation

Updated docs/src/reference/experimental/stubbing.md:

• New "Stubbing foreign functions" section with example
• New "Stub compatibility and lifetime considerations" section warning about lifetime mismatches

Call-outs

• The generic param renaming uses GenericArgs::identity_for_item to get the full substitution (including parent type params), then overrides the function's own params by position. This correctly handles methods on generic types like LocalType<T>::pub_fn.
• Region/lifetime comparison is intentionally relaxed (erased) — the old code also didn't compare lifetimes (MIR body types have regions erased). The difference is now documented.
• The FFI fix only handles local extern blocks. Foreign functions from external crates (e.g., libc::sysconf) already worked via resolve_in_foreign_module when the crate was resolved; the issue was only with local extern "C" declarations.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.