_common.sh: installed and toolkit copies resolve different roots when cwd is inside app/ or apps/<X>/ #489

Found in: the independent review of #487 (2026-10-20), which fixes #420.

When the working directory is inside an app folder (app/ or apps/<X>/), the two copies of _common.sh give different project roots:

• The installed copy (<project>/bin/_common.sh, tier 2) resolves to the project folder.
• The toolkit copy (tier 3) resolves to the app folder. mxtk_walk_root (_common.sh around line 53) checks for a model before it checks for a project folder, so it stops at the first folder that holds the .mpr.

A script run from the toolkit and the same script installed in the project can therefore write their ledgers (look ledger, metrics ledger, BUILD-LOG) to different roots, and the records split.

Expected: when the walk finds a model in a folder whose parent app/ (or grandparent apps/<X>/) is a project folder, it returns that project folder. Both copies then agree, on single-tree, two-tree and multi-app layouts. Add a fixture case for cwd in app/javasource and in apps/A/ with both copies.

This was already true on master before #487; it is not a regression from that PR.

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