Make assignment construction fallible (#137)

[leynos]

Summary

This branch makes manual ClusteringResult construction explicitly fallible,
so invalid cluster identifiers produce NonContiguousClusterIds rather than a
public-API panic. The CPU pipeline retains its crate-private convenience
constructor because it establishes contiguous labels by construction.

Closes #137.

Review walkthrough

• Start with the result constructor boundary, which makes the panicking helper CPU-only and crate-private while retaining try_from_assignments as the public API.
• Then review the public API surface test and the error-variant regression table.
• Finish with the users' guide and the design note.

Validation

• make check-fmt: passed
• make lint: passed (Rustdoc, Clippy, and Whitaker)
• make test: passed (1,087 tests)
• make markdownlint: passed
• make nixie: passed
• cargo test --workspace --doc: passed (5 doctests)
• coderabbit review --agent: 0 findings

References

• Lody session

Summary by Sourcery

Make manual clustering-result construction explicitly fallible while preserving a CPU-internal convenience path for guaranteed-valid assignments.

New Features:

• Make manual ClusteringResult construction use a public fallible API that reports invalid cluster identifiers through NonContiguousClusterIds.
• Retain the panicking assignment constructor as an internal CPU-only convenience for pipelines that guarantee contiguous labels.

Bug Fixes:

• Prevent invalid manually supplied cluster identifiers from causing panics through the public API.

Enhancements:

• Update CLI usage, examples, and user-facing design guidance to use and document fallible result construction.

Documentation:

• Document the fallible construction contract and clarify the internal scope of the panicking convenience constructor.

Tests:

• Add a compile-fail API-surface test confirming the panicking constructor is not publicly accessible.
• Expand result-construction regression coverage for invalid cluster identifier cases.

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[sourcery-ai]

Reviewer's Guide

The PR changes manual ClusteringResult construction from a public panicking API to the public fallible try_from_assignments, confines the panicking helper to the CPU pipeline, adds API and regression tests, updates documentation and callers, and records unrelated validation investigations.

Sequence diagram for fallible clustering result construction

sequenceDiagram
    participant Caller
    participant ClusteringResult
    participant CpuPipeline

    Caller->>ClusteringResult: try_from_assignments(assignments)
    ClusteringResult-->>Caller: Ok(ClusteringResult)
    ClusteringResult-->>Caller: Err(NonContiguousClusterIds)
    CpuPipeline->>ClusteringResult: from_assignments(assignments)
    ClusteringResult-->>CpuPipeline: ClusteringResult

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File-Level Changes

Change	Details	Files
Make manual clustering-result construction explicitly fallible while retaining an internal CPU-only fast path.	Restrict the panicking constructor to the cpu feature and crate visibility. Update callers and examples to use try_from_assignments and handle validation errors. Document the public fallible construction contract and internal CPU invariant.	chutoro-core/src/result.rs chutoro-cli/src/cli/commands.rs chutoro-cli/src/cli/tests.rs docs/users-guide.md docs/chutoro-design.md
Add regression coverage for the constructor’s public API boundary and invalid-identifier error behavior.	Add a trybuild compile-fail test proving from_assignments is not externally accessible. Update assignment-based tests to exercise the fallible constructor. Document duplicate identifiers alongside missing-zero, gap, and overflow error cases.	chutoro-core/tests/result_api_surface.rs chutoro-core/tests/trybuild/clustering_result_from_assignments_private.rs chutoro-core/tests/trybuild/clustering_result_from_assignments_private.stderr chutoro-core/tests/chutoro.rs docs/chutoro-design.md
Record investigation plans for unrelated validation blockers encountered during the change.	Capture hypotheses and isolation steps for benchmark smoke-test timeouts. Capture CPU-disabled fixture diagnostic drift caused by warnings promoted to errors.	docs/debugging/debugging-plan-2026-08-24T153310+0200.md docs/debugging/debugging-plan-2026-08-24T155013+0200.md

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#137	Make unchecked or manually constructed ClusteringResult values use an explicitly fallible public constructor, so invalid cluster identifiers return a typed error instead of causing a public API panic.	✅
#137	Remove or restrict the panicking from_assignments constructor so it cannot be used as a public API for invalid input.	✅
#137	Add regression coverage and documentation for invalid, non-contiguous, and overflowing assignment inputs through the public fallible API.	✅

Possibly linked issues

• Make ClusteringResult assignment construction explicitly fallible #137: The PR makes try_from_assignments the public constructor, restricts from_assignments, and adds API regression coverage.

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