fix(chain)!: reject changesets that replace the genesis block

[Brijesh-Thakkar]

Fixes #2309

Description

LocalChain::apply_changeset was the only mutation entry point that did not protect the genesis block. A ChangeSet containing (0, Some(hash)) with a hash different from the chain's genesis made apply_changeset_to_checkpoint rebuild the chain via LocalChain::from_blocks without comparing against the old genesis, so the chain silently moved onto a different genesis. apply_update (via merge_chains), insert_block and disconnect_from already refuse this.

This adds a check at the top of apply_changeset_to_checkpoint. If the changeset has a height-0 block whose hash differs from the chain's genesis, it returns the new ApplyBlockError::CannotReplaceGenesis { expected } before anything is applied, so the chain is left unchanged. A matching genesis entry still applies, and from_changeset is unaffected because it seeds genesis from the same changeset.

Notes to the reviewers

• ApplyBlockError is not #[non_exhaustive], so adding a variant is a breaking change for exhaustive matches. I can add #[non_exhaustive] or use a different approach if you prefer.
• merge_chains already rejects a differing genesis before reaching this code, so its new match arm only satisfies the exhaustive match and maps to CannotConnectError { try_include_height: 0 }.
• The check uses get(0), which only returns checkpoints with data. This matches the !is_placeholder() check in merge_chains. A LocalChain cannot hold a placeholder genesis, so I could not write a test for that case.
• (0, None) already fails with MissingGenesis and is unchanged. A test pins that behavior.
• The error carries only expected, consistent with PrevBlockhashMismatch.
• A closed PR (fix(chain): reject a changeset that replaces the genesis block #2311) proposed a similar approach.
• AI assistance: I used Claude Code to help analyze the issue and draft the fix. I reviewed the diff and ran the checks myself.
• I ran fmt, clippy, the workspace tests, the bdk_chain feature and no_std builds, cargo doc with -D warnings, and the 1.85.0 MSRV build and tests locally. The other crates' per-feature builds are left to CI.

Changelog notice

Changed

• LocalChain::apply_changeset returns the new ApplyBlockError::CannotReplaceGenesis, and leaves the chain unchanged, when the changeset would replace the genesis block. LocalChain::apply_changeset silently replaces the genesis block #2309
• Breaking: ApplyBlockError has a new variant, so exhaustive matches must handle it.

Checklists

All Submissions:

• I followed the contribution guidelines

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

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