meta: reap detached trees that contain immutable files

[solracsf]

Clone copies the source node wholesale, flags included, so cloning a tree with a chattr +i file produces immutable copies. When a clone is interrupted the top directory stays detached, and its cleanup goes through emptyDir -> BatchUnlink/Rmdir, which refuse entries carrying FlagImmutable or FlagAppend (uid 0 does not bypass that). CleanupDetachedNodesBefore then failed on the tree on every gc --delete, logging an error each run while the tree kept its slice references alive, so those blocks could never be reclaimed.

Mark the context used to remove a detached tree and let the flag checks in doBatchUnlink and doRmdir stand down for it, in all three engines. A detached tree is unreachable and this cleanup is its only way out.

Removal of reachable files is untouched: the marker is set only by doCleanupDetachedNode.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.03%. Comparing base (53835e2) to head (86f46e0).

Files with missing lines	Patch %	Lines
pkg/meta/sql.go	40.00%	0 Missing and 3 partials ⚠️
pkg/meta/tkv.go	40.00%	0 Missing and 3 partials ⚠️
pkg/meta/redis.go	60.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7407       +/-   ##
===========================================
+ Coverage   34.47%   45.03%   +10.56%     
===========================================
  Files         140      177       +37     
  Lines       48322    58723    +10401     
===========================================
+ Hits        16658    26448     +9790     
+ Misses      29984    29212      -772     
- Partials     1680     3063     +1383     

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