Represent nested workspaces and subproject target slices

[godofecht]

Problem

Large repos have nested packages and workspace-like layouts. A single root module map is not enough.

Needed

• Represent subprojects/nested build packages.
• Allow target selection inside a repo without building every sample/tool.
• Record package-local roots and relative output locations.
• Keep diagnostics scoped to the subproject that failed.

Corpus drivers

• MicroZig has many nested build packages.
• zig-gamedev has samples/experiments with optional dependencies.
• Mach has examples/editor/modules.

Done when

• Corpus manifests can name a target slice like samples/minimal_* or port/<chip> without overbuilding the repo.
• --build and future --parity reports include the selected target path.