checker,cgen: allow embedded interface upcasts

[vincenzopalazzo]

Summary

• allow implicit interface-to-interface upcasts when the source interface embeds the target interface
• generate implicit interface conversions in cgen the same way explicit as casts do
• make interface type-name helpers return string so minimal interface cast programs do not depend on tos3
• add a regression test for fn-call, assignment, and explicit as cases from implicit cast inconsistency #26759

Root cause

type_implements rejected all interface-to-interface implicit conversions, even when the source interface explicitly embedded the target one. The C backend also only handled interface-to-interface conversions for explicit as casts, so implicit conversions were passed through as the wrong interface type.

Verification

• ./vnew vlib/v/tests/interfaces/interface_embedding_implicit_upcast_test.v
• exact issue reproducer from implicit cast inconsistency #26759 now compiles with ./vnew
• ./vnew vlib/v/tests/interfaces/interface_runtime_conversions_test.v
• ./vnew -silent vlib/v/compiler_errors_test.v
• ./vnew -silent vlib/v/gen/c/coutput_test.v

Notes

• ./vnew -silent test vlib/v/ is not locally clean in this workspace because of unrelated pre-existing worktree changes in vlib/v/checker/containers.v, vlib/v/gen/c/fn.v, and vlib/v/tests/interface_string_ref_arg_test.v.

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[JalonSolov]

Close/re-open for CI run.

[JalonSolov]

Several conflicts to resolve.

[vincenzopalazzo]

@JalonSolov Fixed in 6e6d0ed. Merged current master into this branch to resolve the conflicts and reran local verification on top of the merge (vlib/v/tests/interfaces/interface_embedding_implicit_upcast_test.v, vlib/v/tests/interfaces/interface_runtime_conversions_test.v, vlib/v/gen/c/coutput_test.v, vlib/v/compiler_errors_test.v, and test vlib/v/; the VLS tests in the broad run needed the repo root on PATH because they shell out to v directly, and they passed after rerunning under that expected local setup).

[JalonSolov]

One more conflict (due to other PR merge).

[vincenzopalazzo]

@JalonSolov Fixed in 4c7deb9. Merged current master into this branch to resolve the typeof_test.v conflict (upstream landed an expanded version of the #26704 typeof test that overlapped with the helper our branch had; took the upstream version since it subsumes ours). Reran local verification: all 80 interface tests, typeof_test.v, coutput_test.v, and compiler_errors_test.v pass.

[vincenzopalazzo]

@JalonSolov Squashed all commits into a single one and rebased on current master (0ecfb00). History is clean now -- one commit, no merge commits, no conflicts.