[bug] _common.sh root search climbs to any ancestor .mpr in a multi-app layout, so ledgers land in the wrong folder #420

Symptom. In a multi-app layout (apps/<X>/X.mpr), instruments that source project-bin/_common.sh can resolve the project root to an ancestor folder. Look-ledger seen rows then land in another folder's ledger, possibly another project's.

Cause. Tier 2 of the root search only accepts an .mpr at <root>/ or <root>/app/. A multi-app project fails tier 2, and tier 3 walks up from $PWD and takes the first ancestor that holds any .mpr. Repro: a project at /tmp/x/stray/proj with apps/A/A.mpr, and a stray .mpr in /tmp/x/stray: the hook writes to stray/.claude/loop/look/. With the stray removed, it climbs further to any .mpr under /tmp. It never reaches the self-parent fallback.

Fix direction. Stop climbing at the first folder holding .claude/ or bin/_common.sh, and probe apps/*/. Best done with the multi-app model key (state follows the model).

Found while reviewing #414.