fix(feature-model): don't mis-report a shared child as a cycle (REQ-269)

[avrabe]

REQ-269 — shared child (diamond/DAG) no longer mis-reported as a cycle (v0.29.0)

Surfaced by REQ-266's coverage. validate_tree's cycle detection used a single
global visited set + BFS and reported every second encounter of a node as
a cycle. A feature reachable through two parents — a diamond, root -> {a, b}
with a -> shared and b -> shared — was enqueued from both parents, dequeued
twice, and wrongly rejected: cycle detected involving feature shared``. Real
feature models legitimately share a sub-feature under two branches.

Fix

Iterative DFS tracking the current path (recursion stack) vs fully-explored
nodes:

• Re-encounter on the current path → genuine cycle (back edge, a feature that
  is its own ancestor). Still rejected.
• Re-encounter by a disjoint path → shared child, fully explored → skipped,
  not flagged.

Parent link stays last-writer-wins (unchanged, acceptable); only the traversal's
cycle criterion changes.

Tests

• Un-ignored diamond_shared_child_is_not_a_cycle — now passes.
• Added genuine_cycle_is_rejected — root -> mid -> root must still error.

Verification

Full feature-model + variant suites green (proptest 14, lib feature_model
62, variant_gap_check 2, variant_phase2 8); clippy --all-targets
(1.97.0) clean; wasm seam builds — cargo build -p rivet-cli --features wasm
OK (composition core touched, WIT/component unchanged so no cargo-component
rebuild).

Fixes: REQ-269 · Refs: REQ-266

🤖 Generated with Claude Code

[github-actions]

📐 Rivet artifact delta

Change	Count
Added	0
Removed	0
Modified	1
Downstream impacted (depth ≤ 5)	0

Graph

graph LR
  REQ_269["REQ-269"]:::modified
  classDef added fill:#d4edda,stroke:#28a745,color:#155724
  classDef removed fill:#f8d7da,stroke:#dc3545,color:#721c24
  classDef modified fill:#fff3cd,stroke:#ffc107,color:#856404
  classDef overflow fill:#e2e3e5,stroke:#6c757d,color:#495057,stroke-dasharray: 3 3

Loading

Modified

ID	Changes
REQ-269

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[codecov]

Codecov Report

❌ Patch coverage is 97.95918% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rivet-core/src/feature_model.rs	97.95%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Rivet Criterion Benchmarks'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 6ab32f7	Previous: ab9b24c	Ratio
traceability_matrix/1000	59877 ns/iter (± 349)	43980 ns/iter (± 781)	1.36
query/10000	339650 ns/iter (± 5633)	223850 ns/iter (± 972)	1.52

This comment was automatically generated by workflow using github-action-benchmark.