Switch to ldk-fuzzing-corpus repo rather than CI cache

[TheBlueMatt]

Rather than storing our fuzzing corpus in the CI cache, move it to a new repo which anyone can use for their own local fuzzing and can be updated outside of CI with additional seeds.

[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.

[ldk-claude-review-bot]

I've thoroughly re-examined every hunk in the diff, cross-referencing with ci-fuzz.sh and generate_fuzz_coverage.sh. After careful analysis, I have no new issues to report beyond the prior review pass.

Review Summary

No new issues found in this review pass.

Corrections to prior review

Upon re-examination, two of my prior inline comments on the coverage job (.github/workflows/build.yml:87-95) were false positives and should be disregarded:

1. "Coverage job cp -r uses wrong relative path with ../../" — Incorrect. The coverage job uses cp -r "ldk-fuzzing-corpus/rust-lightning/${NAME}" "hfuzz_workspace/${NAME}_target/input", which is the correct relative path from CWD (fuzz/). The ../../ prefix only appears in the fuzz job's ln -sfn, where it is correct (symlink targets resolve relative to the symlink's location, not CWD).

2. "Coverage job gets zero corpus files due to find not following symlinks" — Incorrect. The coverage job uses cp -r to create real directories, not symlinks. The find in generate_fuzz_coverage.sh:98 works correctly on real directories.

Prior comments that remain valid (not repeated)

• .github/workflows/build.yml:274-284 — SIG* crash files staged to wrong path (new-corpus/$NAME vs new-corpus/rust-lightning/$NAME)
• .github/workflows/build.yml:273 — Clone failure causes staging step to crash with git error
• .github/workflows/push-fuzz-corpus.yml:92 — --base master may not match the corpus repo's default branch
• .github/workflows/push-fuzz-corpus.yml:10 — Comments reference master but rust-lightning's default branch is main

Minor observation (not worth an inline comment)

• Coverage job step name at line 87 says "Symlink corpus into hfuzz_workspace" but actually uses cp -r (copy-paste from the fuzz job without updating the name).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.85%. Comparing base (8712856) to head (efa95b4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4588      +/-   ##
==========================================
- Coverage   87.17%   86.85%   -0.32%     
==========================================
  Files         161      161              
  Lines      109260   109260              
  Branches   109260   109260              
==========================================
- Hits        95244    94899     -345     
- Misses      11544    11781     +237     
- Partials     2472     2580     +108     

Flag	Coverage Δ
fuzzing-fake-hashes	26.82% <ø> (-4.38%)	⬇️
fuzzing-real-hashes	20.62% <ø> (-2.32%)	⬇️
tests	86.23% <ø> (-0.01%)	⬇️

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[TheBlueMatt]

Okay, updated to run on PRs properly, but sadly we'll have to actually merge this to test it.

[valentinewallace]

Landing to see if it works

[valentinewallace]

nit: main

[TheBlueMatt]

Seems to work: lightningdevkit/ldk-fuzzing-corpus#3 I have a feeling that repo's git history is going to get...bad

[joostjager]

Wanted to add a few of my strings in lightningdevkit/ldk-fuzzing-corpus#4, but it complains

[TheBlueMatt]

Complains how? If you open a PR that is add-files-only it should get auto-merged daily.

[joostjager]

It says "Merging is blocked. Cannot update this protected ref.". Or is that expected?