docs: update CHANGELOG for v2.1.0

[mason-sharp]

Expand the unreleased section into the full v2.1.0 entry covering all changes since v2.0.0. This release focuses primarily on Merkle tree functionality: CDC drain correctness (silent under-reporting, phantom mismatches, skipped transactions), performance (adaptive bulk rehash, concurrent per-node drains), repairability of mtree-produced diffs for complex column types, and the spock-diff repair. Also notes the new --version flag and the per-node-pair max_diff_rows behaviour change.

[codacy-production]

Up to standards ✅

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 2992c970-cdd8-428f-9a60-5ec605390843

📥 Commits

Reviewing files that changed from the base of the PR and between 4c8178b and 30e68a9.

📒 Files selected for processing (1)

• docs/CHANGELOG.md

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📝 Walkthrough

Walkthrough

The CHANGELOG.md file is updated to replace an Unreleased section with a new v2.1.0 release section, documenting Added, Changed, and Fixed entries related to Merkle-tree/CDC diff and drain capabilities, CLI flags, configuration keys, and various correctness and operability fixes.

Changes

Changelog Update

Layer / File(s)	Summary
Added and Changed release notes docs/CHANGELOG.md	Introduces v2.1.0 section header, new CLI flags (--version/-V, --cdc-timeout), max_diff_rows cap, concurrency behavior for mtree table-diff/update alongside listen, new cdc config keys, and revised CDC drain sub-batching/timeout semantics.
Fixed issues documentation docs/CHANGELOG.md	Documents fixes for divergence under-reporting, tail-range diff coverage, stale diff results, repairability/type normalization, spock-diff mismatches, mtree init ordering, ACE metadata replication, resource leaks, fail-fast on unbuilt trees, older-cluster compatibility, and a no-primary-key drain error.

Poem

A rabbit hops through changelog land,
v2.1.0 notes now planned,
CDC batches drain with care,
stale diffs vanish into air,
🥕 hop, hop — release day's here!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: updating the changelog for the v2.1.0 release.
Description check	✅ Passed	The description is directly related to the changelog update and accurately summarizes the release notes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch prepare-v2.1.0

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