fix(filecow): bound unchanged snapshot layer growth

[yiying-zhang]

This PR addresses the known memory-leak issue.

Summary

• avoid allocating filecow generations when a mapping has no private state to
  preserve;
• reuse the current immutable layer for unchanged child mappings;
• defer layer allocation until Btrfs extent shareability is known, avoiding
  empty generations for unshareable cache;
• expose filecow lifetime counters in /proc/filecow_stats; and
• add a repeated Btrfs snapshot lifetime selftest.

This fixes unbounded layer-chain growth observed across repeated unchanged
container forks. It intentionally does not change tfork userspace cleanup or
latency coordination.

Validation

• booted kernel 7.0.1-filecowfix-pgcachecow #4;
• 100-iteration Btrfs kernel selftest passed;
• 20 isolated live container forks passed inheritance and source/child
  divergence checks;
• 120 inode-level forks left layers_allocated and layers_active flat at 3;
• 100-cycle and 1,000-cycle combined-stack soaks run separately in the final
  integration gate.

Safety

• retains layers while inode/mapping owners remain cached;
• changes allocation/reuse, not page ownership or write-COW semantics;
• preserves the previous kernel in GRUB as a recovery fallback.