refactor: remove unreachable Partition delta-computation interface methods

[carlos-alm]

Summary

• Partition.deltaCPM/.deltaModularityDirected/.deltaModularityUndirected/getCandidateCommunityCount interface methods have zero callers anywhere — computeQualityGain (the only caller of delta computation in the Leiden hot loop) calls standalone diffCPM/diffModularity functions directly instead. Confirmed Partition/makePartition aren't part of the public API (src/index.ts exports nothing from the leiden module).
• Chose removal over wiring the interface methods into the hot loop: the two implementations read through different data-access layers (raw typed arrays vs. a getter interface), and this file has a track record (codegraph communities --drift is non-deterministic across runs on identical source (breaks /titan-gate arch-snapshot comparison) #1734, leiden/partition.ts: computeDeltaModularityDirected exceeds cyclomatic threshold (cyc=11) #1755) of exactly this kind of subtle numerical divergence — verifying "equivalent formula" actually produces "equivalent runtime values" would require the same rigor as those two fixes, for no benefit over just deleting genuinely unreachable code.
• Empirically verified (not trusted from comments) that leiden/partition.ts: computeDeltaModularityDirected exceeds cyclomatic threshold (cyc=11) #1755's regression tests don't constrain this removal — replaced all four functions with throw and ran the full suite; all 3549 tests still passed, proving leiden/partition.ts: computeDeltaModularityDirected exceeds cyclomatic threshold (cyc=11) #1755's "directed modularity delta" test actually exercises a different function (diffModularityDirected in modularity.ts) via the real call path. Corrected that misattributed test comment. Also removed fgetOrZero (added by leiden/partition.ts: computeDeltaModularityDirected exceeds cyclomatic threshold (cyc=11) #1755), now orphaned as a direct consequence — leaving it would ship newly-created dead code from this very change.

Closes #1770

Test plan

• npm test — full suite green (3545 passed, 0 failed)
• npm run lint / npm run typecheck — clean
• Community detection confirmed byte-identical: 36 combinations (file/function level × modularity/CPM × 3 resolutions × 3 seeds) on this repo's own 920-file graph, before/after SHA-256 hashes matched exactly

Filed #1895 for codegraph's own dead-code detector blind spot found during investigation: object-literal property-value references count as liveness without checking if the property is ever invoked (not fixed here, out of scope).

Stacked on #1894 (base branch fix/issue-1769-codegraph-method-attribution) — only the leiden/test diff is this issue's change.

[greptile-apps]

Greptile Summary

This PR removes four dead interface methods (deltaCPM, deltaModularityUndirected, deltaModularityDirected, getCandidateCommunityCount) from the Partition interface and their implementations in partition.ts, along with the fgetOrZero helper in typed-array-helpers.ts that was only consumed by those removed functions. The test file is updated to remove the direct unit tests for the deleted code and to correct a misattributed comment that had claimed the regression test exercised Partition.deltaModularityDirected (which is unreachable) rather than diffModularityDirected in modularity.ts.

• partition.ts: Removes ~160 lines of dead delta-computation functions (computeDeltaModularityUndirected, computeDeltaModularityDirected, computeDeltaCPM, and their helpers) plus the four interface method bindings; all live callers go through computeQualityGain → diffCPM/diffModularity in optimiser.ts instead.
• typed-array-helpers.ts: Removes fgetOrZero (introduced in leiden/partition.ts: computeDeltaModularityDirected exceeds cyclomatic threshold (cyc=11) #1755 solely for the now-deleted functions), keeping the file lean with only fget, iget, u8get, and taAdd.
• leiden.test.ts: Drops the fgetOrZero unit tests (function no longer exists) and rewrites the regression-test comment to correctly describe the actual code path being exercised.

Confidence Score: 5/5

Safe to merge — removes verified dead code with no live callers; all 3549 tests pass and community detection output is byte-identical before and after.

The deleted interface methods have zero callers in the codebase (confirmed by grepping src/), the actual hot-loop computation routes exclusively through standalone diffCPM/diffModularity functions in optimiser.ts, and the PR description documents an empirical proof (replacing the functions with throw left the full test suite green). The only change to surviving code is the test comment correction, which is accurate.

No files require special attention — all three files are straightforward deletions or comment corrections with no logic changes to surviving code paths.

Important Files Changed

Filename	Overview
src/graph/algorithms/leiden/partition.ts	Removes 4 dead interface methods and ~160 lines of delta-computation implementation; remaining code is unchanged and the interface stays internally consistent.
src/graph/algorithms/leiden/typed-array-helpers.ts	Removes fgetOrZero (now orphaned after the partition.ts deletion); no remaining callers exist anywhere in the codebase.
tests/graph/algorithms/leiden.test.ts	Drops fgetOrZero import and its 4 direct unit tests; corrects misattributed comment on the directed-modularity regression test to reflect the actual live call path.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[optimiser.ts\ncomputeQualityGain] -->|quality === 'cpm'| B[cpm.ts\ndiffCPM]
    A -->|quality === 'modularity'| C[modularity.ts\ndiffModularity]
    C -->|graph.directed| D[modularity.ts\ndiffModularityDirected]
    C -->|undirected| E[modularity.ts\ndiffModularityUndirected]

    subgraph removed["Removed dead code (this PR)"]
        F["Partition.deltaCPM()"]
        G["Partition.deltaModularityDirected()"]
        H["Partition.deltaModularityUndirected()"]
        I["Partition.getCandidateCommunityCount()"]
        J["fgetOrZero() in typed-array-helpers.ts"]
    end

    style removed fill:#fee2e2,stroke:#ef4444,color:#7f1d1d

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flowchart TD
    A[optimiser.ts\ncomputeQualityGain] -->|quality === 'cpm'| B[cpm.ts\ndiffCPM]
    A -->|quality === 'modularity'| C[modularity.ts\ndiffModularity]
    C -->|graph.directed| D[modularity.ts\ndiffModularityDirected]
    C -->|undirected| E[modularity.ts\ndiffModularityUndirected]

    subgraph removed["Removed dead code (this PR)"]
        F["Partition.deltaCPM()"]
        G["Partition.deltaModularityDirected()"]
        H["Partition.deltaModularityUndirected()"]
        I["Partition.getCandidateCommunityCount()"]
        J["fgetOrZero() in typed-array-helpers.ts"]
    end

    style removed fill:#fee2e2,stroke:#ef4444,color:#7f1d1d

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[github-actions]

Codegraph Impact Analysis

1 functions changed → 4 callers affected across 2 files

• makePartition in src/graph/algorithms/leiden/partition.ts:378 (4 transitive callers)