fuzz: Add upgrade/downgrade simulation to chanmon_consistency and fix chacha20 build

[Atishyy27]

Closes #4452

This PR introduces upgrade/downgrade serialization coverage to the chanmon_consistency fuzzer as discussed.

Changes:

1. Fuzzer Logic: Modified the reload_node closure in chanmon_consistency.rs. Based on a fuzzer input byte, it randomly appends a dummy/unknown odd-type TLV record (0xBADF00D1) to the serialized ChannelManager byte stream before reloading. This simulates loading data written by a newer version, ensuring we don't drop channels on unknown odd TLVs.
2. Build Fix: Added the missing seek_to_block dummy method to the #[cfg(fuzzing)] block of ChaCha20 in lightning/src/crypto/chacha20.rs. Without this, the fuzzer binaries currently fail to compile on main (Exit Code 1).

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.21%. Comparing base (cb951b4) to head (2358ca7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4499      +/-   ##
==========================================
+ Coverage   86.18%   86.21%   +0.03%     
==========================================
  Files         160      160              
  Lines      107441   107441              
  Branches   107441   107441              
==========================================
+ Hits        92593    92634      +41     
+ Misses      12229    12189      -40     
+ Partials     2619     2618       -1     

Flag	Coverage Δ
tests	86.21% <ø> (+0.03%)	⬆️

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