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.

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