bbssd: add deleted-data-remanence experiment logging

[NiceGuy1313]

Add debug-only instrumentation to the BBSSD FTL to study whether deleted data survives in the emulated device. FTL behavior is unchanged: every addition is gated on the FEMU_EXP_LOG / FEMU_SECRET env vars and only emits logs (to stderr, so it survives the tee pipe unbuffered).

ftl.c:

• EXP_LOG macro + PPA_FMT/PPA_ARG human-readable PPA formatting
• femu_dbg_dump_lpn(): hex/ascii dump of a logical page from the DRAM backend (data lives at LBA offset in mbe->logical_space, not at a PPA)
• femu_dbg_scan_secret(): scan the whole backend for FEMU_SECRET
• content-based watch list so only the secret-bearing LPN/block is logged
• log points: [WRITE] [INVALIDATE:overwrite] [INVALIDATE:trim] [GC_MOVE] [ERASE], plus [SCAN:after_trim] and FEMU_DUMP_LPN-triggered [DUMP]

femu-scripts/run-blackbox.sh:

• forward FEMU_EXP_LOG/FEMU_SECRET/FEMU_DUMP_LPN through sudo (sudo otherwise strips them so getenv() returns NULL inside qemu)

Description

Brief description of changes made.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring

Testing

• I have tested my changes locally
• I have added tests that prove my fix is effective or that my feature works
• All existing tests pass
• I have tested across multiple FEMU modes (if applicable)

FEMU Modes Tested

• BlackBox SSD (BBSSD)
• WhiteBox SSD (OCSSD)
• Zoned Namespace SSD (ZNSSD)
• NoSSD
• Not applicable

Platform Testing

• Ubuntu 20.04/22.04
• Other distributions (specify): ___________
• Build verification completed

Checklist

• My code follows QEMU coding standards
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new compiler warnings
• I have updated documentation if necessary
• No trailing whitespace or C++ style comments in C files

Related Issues

Fixes #(issue number)

Additional Notes

Any additional information, configuration changes, or notes for reviewers.