Locate Robolectric host libraries built by the swiftbuild engine

[piercifani]

Summary

loadPeerLibrary only looked for the Robolectric host library under the native build system's <triple>/debug/ layout. Swift 6.4 made swiftbuild the default SwiftPM engine on Linux, and it writes products to out/Products/Debug-linux-<arch>/ (out/Products/Debug/ on macOS), so every Robolectric test now fails with missing library on CI hosts that moved to a 6.4 toolchain, main included. The loader now tries the native location first, then the two swiftbuild locations, and reports all of them when none exists.

Evidence

Same workflow, same day, on a fork:

• unchanged main: both ubuntu jobs fail with missing library …/x86_64-unknown-linux-gnu/debug/lib*.so (468 occurrences each) https://github.com/piercifani/skip-bridge/actions/runs/35981813831
• main + this commit: ubuntu-24.04 and ubuntu-24.04, nightly-main pass https://github.com/piercifani/skip-bridge/actions/runs/35986700823

Locally (macOS, native engine) the suite still passes 69/69, so the native path is unaffected.

🤖 Generated with Claude Code

[piercifani]

First-party confirmation: today's scheduled run of unchanged main (https://github.com/skiptools/skip-bridge/actions/runs/35982490951) fails both ubuntu jobs with the same missing library signature, so main is red on the hosted runners until this (or an equivalent) lands.

[piercifani]

Superseded by #119, which already adds the same swiftbuild product-folder fallback to loadPeerLibrary (plus the Android build-system change) and is green on all four jobs. Closing this to avoid two fixes for one problem; #120 now stacks on #119 instead.