feat(version): add GROVE_V4, behaviourally identical to V3

[QuantumExplorer]

Why

GROVE_V3 is live. A fix that changes an accepted/rejected outcome, a committed root hash, or a tracked cost therefore cannot be applied unconditionally — nodes carrying it would diverge from nodes that do not. Today there is nowhere for such a fix to land, which has left several stuck:

Fix	Why it cannot land ungated
#776 — bare Reference overwrite skips indexed secondary cleanup	costs an extra stored-element read on every reference overwrite (+1 seek, +79 storage_loaded_bytes, measured by the refresh-reference cost tests); references over plain trees are shipped
Batch DeleteTree trusts the caller-declared tree type when selecting cleanup namespaces	reading the stored element instead fixes an indexed type-confusion and a live CommitmentTree wrong-emptiness-path bug, but adds a read to a released path
#726, #730, #732, #734, #739	currently gated on v3; need re-gating before they can merge

What this does

Adds the version and nothing else. Every method-version slot is copied from V3 unchanged, so activating protocol version 4 today is a no-op. Each future gate is a deliberate, separately-reviewable slot bump — the same pattern add_element_on_transaction: 1 already uses in v3.rs.

Landing it standalone rather than inside a feature branch means all six of the PRs above can target it independently, and avoids a multi-way conflict in v4.rs.

Verification

Registering V4 changes what GroveVersion::latest() resolves to, and the whole test suite defaults to latest — so "identical to V3" needed checking, not assuming.

The full workspace suite passes with V4 as latest (2459 grovedb + 705 merk + the rest). The only two failures were the version registry's own self-describing tests, grove_version_latest_returns_v3 and grove_versions_count, both updated here. That the behavioural suite is untouched is the evidence this change is inert.

Also adds grove_v4_is_behaviourally_identical_to_v3_until_a_gate_is_added, which compares every slot and fails the moment one is bumped — the intended prompt to document a gate rather than let V4 accrete behaviour silently.

Note for reviewers

This does not activate anything. Platform must adopt protocol version 4 for any future gate to take effect, and until then V1–V3 behaviour is entirely unchanged. Worth deciding the full gate list before activation so "what changes when we turn on 4?" has one answer.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Added protocol version 4 support.
  • GroveDB now recognizes protocol V4 as the latest available version.
  • Introduced V4 version definitions for GroveDB and Merk operations, initially preserving V3 behavior where no explicit changes are gated.

• Tests

  • Expanded version-selection coverage to include V4.
  • Added regression checks confirming V4’s initial behavior matches V3.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 22f93991-6398-4e0a-9aab-ab4291f92869

📥 Commits

Reviewing files that changed from the base of the PR and between 98c8e75 and 543fb39.

📒 Files selected for processing (1)

• grovedb-version/src/tests.rs

📝 Walkthrough

Walkthrough

Protocol V4 is added as a public Grove version constant, registered in GROVE_VERSIONS, and selected as the latest version. Tests verify the registry size, latest protocol, and V4 slot compatibility with V3.

Changes

Grove protocol V4

Layer / File(s)	Summary
Define and register protocol V4 grovedb-version/src/version/v4.rs, grovedb-version/src/version/mod.rs	Defines GROVE_V4 with explicit Grovedb and Merk method-version slots, exports the module, and adds V4 to GROVE_VERSIONS.
Validate V4 selection and compatibility grovedb-version/src/tests.rs	Verifies V4 is the latest registered protocol, updates the registry count to four versions, and compares V4 slots with V3.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• dashpay/grovedb#669: Adds code paths gated by the Merk proof version slots introduced for V4.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly states the main change: adding GROVE_V4 and noting it matches V3 behavior.
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 docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/grove-v4

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.49%. Comparing base (cc60ac9) to head (543fb39).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #777   +/-   ##
========================================
  Coverage    91.49%   91.49%           
========================================
  Files          240      240           
  Lines        67763    67763           
========================================
  Hits         62003    62003           
  Misses        5760     5760           

Components	Coverage Δ
grovedb-core	89.07% <ø> (ø)
merk	92.32% <ø> (ø)
storage	86.52% <ø> (ø)
commitment-tree	96.05% <ø> (ø)
mmr	96.79% <ø> (ø)
bulk-append-tree	89.82% <ø> (ø)
element	97.38% <ø> (ø)

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