fix(p2p-12): prefer deepest matching mount lookup (#671)

[jlon]

Summary

• Fix mounted-path lookup to prefer the deepest matching mount.
• Keep this as a detached side PR; later P2P slices do not depend on broader unified refactors.

Design

• Limit scope to the independently mergeable boundary bug in mount selection.
• Exclude write-type, consistency, and route changes that depend on later mount-model evolution.

Implementation

• Update mount cache lookup to choose the deepest valid match for overlapping mounts.
• Leave cache sync and unified filesystem route changes out of this PR.

Verification

• Verify overlapping mounts resolve to the deepest matching mount.
• Verify non-overlapping mount selection remains unchanged.

[jlon]

Part 12/17 of the split P2P series that supersedes #671.

This PR targets , so the visible diff is cumulative. Review the slice described in this PR body instead of treating the whole diff as new.

Detached side PR. Review after #752; later slices do not depend on this one.

[jlon]

Review note: this is part 12/17 of the split P2P series that supersedes #671.

This PR targets main, so the visible diff is cumulative. Please review the slice described in this PR body.

Detached side PR. Review after #752; later slices do not depend on this one.