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DF-0875 FIX VALIDATION — patched kernel (#1, sha256 87ad15e5...)
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POC: create hammer2 image, newfs, corrupt sroot_blockset.blockref[0].data_off
     to 0x004000000000000e (1 PB offset, radix 14), recompute CRC32C, mount.

PATCHED KERNEL RUN (the "after"):
--- patch image ---
copy 0 @0x0: bref[0].type=1 data_off=0x000000000140000a
copy 0: PATCHED data_off=0x004000000000000e ICRC1=73635277 ICRC0=6c269868 ICRCVH=c45a6a2f
patched 1 volume-header copies
1 copies OK, 0 BAD
copy 0: data_off=0x004000000000000e ICRC0 exp=6c269868 got=6c269868 OK | ICRC1 exp=73635277 got=73635277 OK | ICRCVH exp=c45a6a2f got=c45a6a2f OK
--- MOUNT (expect clean error, NO panic) ---
mount: Invalid argument
MOUNT_RC=1
--- GUEST STILL UP? ---
 8:54PM  up 49 secs, 0 users, load averages: 0.12, 0.04, 0.01
--- dmesg relevant ---
hammer2_ondisk: "/dev/vn0" zone=0 id=0 offset=0x0000000000000000 size=0x0000000010000000
hammer2_get_volume: no volume for offset 0x0040000000000000
hammer2_chain_load_data: I/O error 004000000000000e: 5
hammer2_mount: error I/O Error reading super-root

Second run (determinism):
=== MOUNT RUN 2 ===
mount: Invalid argument
MOUNT_RC=1
 8:55PM  up 1 min, 0 users, load averages: 0.10, 0.04, 0.01

Regression check (uncorrupted image still mounts + writable):
MOUNT_RC=0
WRITE_OK

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BEFORE/AFTER CONTRAST
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BEFORE (unpatched #0, sha256 built Jul 2):
  panic: no volume for offset 0x0040000000000000
  hammer2_get_volume() at hammer2_get_volume+0x4b
  _hammer2_io_getblk() at _hammer2_io_getblk+0x477
  _hammer2_io_bread() at _hammer2_io_bread+0x17
  hammer2_chain_load_data() at hammer2_chain_load_data+0x2a5
  hammer2_chain_lock() at hammer2_chain_lock+0xde
  => kernel panic, guest dead in DDB

AFTER (patched #1, sha256 87ad15e5ea59faa0aec00ace3681bcd92b2c5b19c9b0ff7090db75be657d30a1):
  hammer2_get_volume: no volume for offset 0x0040000000000000  (diagnostic kprintf)
  hammer2_chain_load_data: I/O error 004000000000000e: 5       (EIO propagated)
  hammer2_mount: error I/O Error reading super-root             (clean EINVAL)
  mount: Invalid argument                                       (MOUNT_RC=1)
  => guest stays UP, no panic, error handled gracefully

=> FIX VALIDATED: panic eliminated, error returned cleanly, no regression.