Fix EventColumnView._byKey unsafe publication; add sort tie-break tests

[jschick04]

What — Fixes an unsafe-publication data race in EventColumnView._byKey and adds two direct sort-kernel regression tests.

Why — ResolveByKey published the lazily-built _byKey dictionary via _byKey ??= BuildByKey() with no memory barrier — unlike every sibling M6 cache (TemplateFieldSchema, CombinedColumnView._index, the column store/pool) which use Volatile/Interlocked/Lock. Under concurrent ResolveByKey, a reader could observe the non-null reference before the dictionary's internal writes are visible on a weak-memory CPU (ARM64) — yielding a wrong/absent result or a throw.

Fix — Publish via Volatile.Read + Interlocked.CompareExchange(ref _byKey, byKey, null) (mirrors TemplateFieldSchema.GetOrBuildIndex). BuildByKey is idempotent over the immutable order, so a benign build-race is harmless.

Tests — grouped group/within-order negation (pins the two -Math.Sign sites independently of the AoS parity oracle); full-tie corpus with a reversed survivor input, so the ascending physical-index tie-break must actively reorder regardless of descending (mutation-verified: the test fails when WithIndexTieBreak is broken).

Validation — Full EventLogExpert.Runtime.Tests: 1641/1641 pass. Surfaced by a post-release audit of the M6 columnar store; the race is latent today (ResolveByKey has no production caller yet — only parity tests).

[Copilot]

Pull request overview

Addresses a potential unsafe-publication race in EventColumnView.ResolveByKey by changing how the lazily built _byKey map is published under concurrency, and adds focused regression tests that pin deterministic sort tie-break behavior in the column-direct sort kernel.

Changes:

• Publish EventColumnView._byKey with Volatile.Read + Interlocked.CompareExchange to ensure safe visibility on weak-memory CPUs under concurrent ResolveByKey.
• Add a full-tie corpus test to ensure the final physical-index tie-break actively reorders even when the primary sort is descending.
• Add a grouped-chain test matrix that independently verifies group-order negation vs within-group-order negation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/Unit/EventLogExpert.Runtime.Tests/LogTable/ColumnDirectSortKernelTests.cs	Adds two direct regression tests covering index tie-break and grouped-chain negation behavior.
src/EventLogExpert.Runtime/LogTable/EventColumnView.cs	Fixes unsafe publication of the lazily built _byKey dictionary via Volatile + Interlocked.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.