Fix SwiftPM product path discovery across toolchains

[Reedtrullz]

Summary

• Replace the hard-coded SwiftPM product directory with SwiftPM's selected --show-bin-path, while retaining an explicit SWIFT_PRODUCTS_DIR override.
• Update the Makefile trust regression to cover the toolchain-independent lookup.

Validation (from the execution ledger)

• Focused Makefile trust tests: 4 passed, 0 failed.
• Full make verify recipe completed through source tests, warnings-as-errors build, release app assembly, plist/codesign checks, and exact product identifiers; the log recorded 1,344 tests with 0 failures and app assembly from .build/out/Products/Release without an override.
• Clean-tree post-commit make app CONFIGURATION=release SIGNING_IDENTITY=-: exit 0.

Scope and blockers

• Source ref: codex/cleanup-vifty-swift-products-dir-2026-08-23 at 9e411b4a00d12dc691fd05471442b552954d66dc.
• This is a local candidate commit, not a release: no privileged administrator environment readback, signed/notarized public artifact, hardware/manual acceptance, merge, or deploy was performed.
• Administrator governance check remains blocked by the committed gh toolchain SHA policy.
• GitHub Actions CI is expected to run for this draft PR; no CI result is being claimed before it reports.