feat(fuzz): add fuzzing harness for LocalChain with CI job

[erickcestari]

Description

Adds an initial fuzzing harness for bdk_chain and a CI job that runs it.

Two fuzz targets in a new fuzz/ crate:

• local_chain_apply_update: builds a LocalChain<BlockHash> and applies random sequences of apply_update, insert_block, disconnect_from, apply_header(_connected_to), including updates derived from the chain's own tip to hit the merge_chains eq_ptr fast path.
• local_chain_apply_update_header: fuzzes LocalChain<Header>, exercising placeholder resolution in apply_update and CheckPoint::entry_iter via a virtual chain of linked headers with random reorgs.

After every operation, the harness asserts that a successful op's changeset reconstructs the post-state from the pre-state, a failed op leaves the chain untouched, and chain invariants hold (strictly decreasing heights, genesis present, is_block_in_chain consistency).

Supports AFL, honggfuzz, and libFuzzer via cargo features. The CI job runs each fuzzer on each target for 5 minutes as a smoke test.

Notes to the reviewers

• The fuzz crate is its own workspace (publish = false) so fuzzer deps don't affect the main workspace.

Changelog notice

• Added a fuzzing harness for LocalChain (AFL, honggfuzz, libFuzzer) with a CI job.

Coverage report

cc @oleonardolima

[codecov]

Codecov Report

❌ Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.31%. Comparing base (5108b5c) to head (8f4d293).

Files with missing lines	Patch %	Lines
crates/chain/src/local_chain.rs	16.66%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2239      +/-   ##
==========================================
- Coverage   78.36%   78.31%   -0.05%     
==========================================
  Files          30       30              
  Lines        5945     5950       +5     
  Branches      281      283       +2     
==========================================
+ Hits         4659     4660       +1     
- Misses       1210     1213       +3     
- Partials       76       77       +1     

Flag	Coverage Δ
rust	78.31% <16.66%> (-0.05%)	⬇️

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[oleonardolima]

It's still missing an updated CI, fixing the conflicts and running each target+harness for an x cycles instead of time, also it requires a proper README.md for users on how to run the fuzzer, and how to add new targets.

[luisschwab]

@erickcestari needs rebase

[luisschwab]

Put bdk_chain targets under a chain folder so we can keep it neat when adding targets from other crates.