CI: per-test-binary DDS domains + a ReleaseSmall lane

[sqt]

Motivation

The ARM64 / DebugAllocator / TSan CI lanes flake intermittently: zig build test runs ~29 participant-creating test binaries in parallel and every one binds domain-0's fixed RTPS ports (SPDP multicast 7400, metatraffic unicast), so under a slow lane the losers hit error.BindFailed, discovery stalls, and a loopback test times out. Separately, tcp_transport_test's reconnect-generation assertion depended on a fixed 100 ms sleep that loses a TCP-settling race under load. And -OReleaseSmall was never built or run anywhere (a roadmap CI-coverage gap).

Description of changes

• Unique domain per test binary. New test/support/domain.zig; build.zig assigns each test binary's Run step a distinct ZZDDS_TEST_DOMAIN_BASE (per-lane counter, via a small addTestRun helper). The ~24 DCPS/C-ABI tests call test_domain.get() instead of hardcoding domain 0 — distinct domains map to disjoint port sets (250-port stride), so parallel binaries never contend. loopback_test / wlp_loopback_test, which hand-wire the stack, also thread the domain through their UdpTransport.init / SpdpSedpDiscovery.init calls.
• tcp_transport_test reconnect race. The "connectionGeneration increments on reconnect" test now drives the reconnecting send() until the new TcpConnection is observed (5 s deadline) instead of sleeping a fixed 100 ms — same invariant asserted, no magic constant.
• ReleaseSmall lane. New zig build test-release-small runs the unit suite at -OReleaseSmall, forcing the LLVM backend to work around a Zig 0.16 self-hosted-x86_64 codegen bug (read-only globals emitted without alignment at that optimize level; fixed on Zig 0.17 master). Wired into run_deterministic_matrix.py and release.yml (Linux x86_64). Revert to the plain self-hosted backend at the Zig 0.17 bump.
• Docs. docs/roadmap.md CI section and docs/design/ci-platform-coverage-expansion.md updated; CHANGELOG.md gains a 2026-08-29 entry.

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Greptile Summary

The PR isolates participant-creating test binaries onto separate DDS domains, replaces a timing-dependent TCP reconnect check with generation polling, and adds LLVM-backed ReleaseSmall coverage.

• Adds a validated environment-based test-domain helper and threads it through socket-creating DCPS/C-ABI tests.
• Adds per-lane domain assignment in the normal, ReleaseSmall, and TSan build graphs.
• Adds ReleaseSmall coverage to the deterministic matrix and Linux release workflow.
• Updates TCP reconnect testing and supporting CI documentation.

Confidence Score: 5/5

The PR appears safe to merge; no concrete blocking or independently actionable non-blocking issue was identified.

The domain helper validates its input and is consistently propagated through the relevant networked tests, while the new reconnect check observes an actual connection-generation change and the ReleaseSmall lane is wired through the intended LLVM-backed graph.

Important Files Changed

Filename	Overview
build.zig	Adds per-binary domain assignment and a complete LLVM-backed ReleaseSmall test lane without an identified correctness issue.
test/support/domain.zig	Parses, validates, and process-caches a bounded DDS test domain with a safe nonzero fallback.
test/transport/tcp_transport_test.zig	Replaces a fixed reconnect delay with bounded polling of the transport's genuine connection-generation signal.
scripts/run_deterministic_matrix.py	Adds the ReleaseSmall build step to local and CI deterministic validation.
.github/workflows/release.yml	Runs the LLVM-backed ReleaseSmall suite on the exercised Linux x86_64 release lane.
test/dcps/loopback_test.zig	Consistently threads the assigned domain through UDP transport, discovery, and participant creation.
test/dcps/wlp_loopback_test.zig	Keeps both loopback participants within the assigned domain while separating this binary from parallel tests.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  B[Zig build lane] --> C[Per-lane domain counter]
  C --> R1[Test binary Run step]
  C --> R2[Test binary Run step]
  R1 --> D1[ZZDDS_TEST_DOMAIN_BASE = N]
  R2 --> D2[ZZDDS_TEST_DOMAIN_BASE = N + 1]
  D1 --> P1[DDS participants and discovery]
  D2 --> P2[DDS participants and discovery]
  P1 --> U1[Distinct RTPS UDP port range]
  P2 --> U2[Distinct RTPS UDP port range]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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