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.

cc @oleonardolima

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.65%. Comparing base (6d03fc3) to head (86fdfd8).

Additional details and impacted files

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  Coverage   78.65%   78.65%           
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  Files          30       30           
  Lines        5909     5909           
  Branches      279      279           
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  Hits         4648     4648           
  Misses       1185     1185           
  Partials       76       76           

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