add weekly corpus fuzzing CI and corpora script

[caarloshenriq]

Add a weekly GitHub Actions workflow that fuzzes every target seeded from payjoin/qa-assets, minimizes the output with -merge=1, and opens a PR to qa-assets with the refreshed corpus for review.

The workflow has three jobs:

• setup: lists all fuzz targets via cargo fuzz list
• fuzz: runs each target in parallel for 5 minutes seeded from
  qa-assets, uploads the resulting corpus as an artifact
• merge: downloads all artifacts, minimizes with corpora.sh refresh, and opens a PR to qa-assets via QA_ASSETS_PUSH_TOKEN

Also add fuzz/corpora.sh with three subcommands:

• seed: copies corpus from qa-assets into fuzz/corpus//
• refresh: minimizes each corpus with cargo fuzz -merge=1
• push: commits the refreshed corpus, pushes to a branch, and opens a PR to qa-assets for human review

Pull Request Checklist

Please confirm the following before requesting review:

• I have disclosed my use of
  AI
  in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Coverage Report for CI Build 33114665563

Coverage remained the same at 86.719%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	16459
Covered Lines:	14273
Line Coverage:	86.72%
Coverage Strength:	343.73 hits per line

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💛 - Coveralls