[ctreffs]

A family created after its entities was filled by walking componentIdsByEntity.keys, and Nexus.EntitiesIterator did the same. Swift seeds Dictionary hashing per process, so the order changed between identical runs. In the Fireblade engine this made lockstep frame captures differ at coplanar faces (fireblade-engine/fireblade#287); with SWIFT_DETERMINISTIC_HASHING=1 the captures were pixel-identical.

Both paths now sort by identifier, as Nexus+Snapshot and Nexus+Clone already do. The guarantee is a deterministic order, one that depends only on the sequence of operations, not ascending order: UnorderedSparseSet removal swaps the last member into the gap. The cost is one sort per family creation and per entities iteration. Not adopted: OrderedDictionary (new dependency, O(n) removal on every destroy) and per-family ordering options (no consumer needs them).

• New NexusDeterministicOrderTests (256 entities) fail before the change and pass after.
• make test: 199 tests pass. make lint: no new warnings in touched files.

Not changed: update(familyMembership entityId:) still walks familyMembersByTraits in hash order. That affects only the order of FamilyMemberAdded events across families, not member order.

https://claude.ai/code/session_01UaLGUVakHTRSr8q7EMhME5

[ctreffs]

Performance

Measured with the benchmark package (Benchmarks/, now on the same ordo-one/benchmark 1.38 setup as fireblade-engine/math), 10,000 entities with 5 components each, Apple Silicon, release build. Baselines: master sources vs this branch, same benchmark code.

Benchmark	What it measures	master p50	branch p50	Change
FamilyCreation	first family(requiresAll:excludesAll:) call on a populated nexus (once per family)	2,943 µs	3,224 µs	+281 µs (+9.5%), +6 mallocs
EntitiesIteration	full makeEntitiesIterator() pass	26 ms	26 ms	none measurable
TypedFamilyEntities	per-frame family iteration (unchanged code)	28 ms	28 ms	none

The only measurable cost is the sort when a family is first created; repeated family(...) calls return early in onFamilyInit, and per-frame iteration is untouched.

Reproduce from Benchmarks/:

git checkout master -- ../Sources/FirebladeECS/Nexus+Entity.swift ../Sources/FirebladeECS/Nexus+Internal.swift
swift package --allow-writing-to-package-directory benchmark baseline update master --filter 'FamilyCreation|EntitiesIteration|TypedFamilyEntities'
git checkout HEAD -- ../Sources
swift package --allow-writing-to-package-directory benchmark baseline update branch --filter 'FamilyCreation|EntitiesIteration|TypedFamilyEntities'
swift package benchmark baseline compare master branch

[ctreffs]

Membership churn

Added FamilyChurn: on an existing family over 10,000 entities, each iteration removes and re-adds Velocity on 1,000 members (they leave and rejoin) and creates and destroys 1,000 matching entities.

Benchmark	master p50	branch p50	Change
FamilyChurn	33 ms	33 ms	none (mallocs 260K / 260K)
FamilyCreation (re-run)	2,927 µs	3,222 µs	+295 µs (+10%)
EntitiesIteration (re-run)	26 ms	26 ms	none
TypedFamilyEntities (re-run)	28 ms	28 ms	none

As expected from the code: component add/remove and destroy go through update(familyMembership: entityId), which this PR does not touch. The sorted path update(familyMembership: traits) has one caller, onFamilyInit, and runs once per family.

Reproduce: same steps as above with --filter 'FamilyChurn|FamilyCreation|EntitiesIteration|TypedFamilyEntities'.