Green the native V8 CI lanes: standalone runner, gtest discovery, gatsby (ECO-416)

[Arshia001]

Stacked on #132. Greens the native v8-linux / v8-macos lanes, which have been red on every stack branch since the native-parity CI was added in b93913d7 (main never runs these steps, so main stays green). Four independent, pre-existing CI-infra issues — none in EdgeJS runtime code; EdgeJS passes every app on both lanes.

Fixes

1. Standalone runner — deterministic (v8-linux + v8-macos).
The native standalone-build-test target left SYMLINK_TARGET unset, so standalone-build-test.js fell back to its default runner build-edge-quickjs-cli/edge — a QuickJS binary the native V8 job never builds → runner target is not executable. Pin the runner to the native binary, mirroring standalone-build-test-quickjs-native.

2. gtest discovery flake (v8-macos "Build edge").
gtest_discover_tests runs each freshly-built test binary at build time to enumerate cases; on macOS runners that intermittently emits empty JSON → ParseTestList.cmake fails the whole build. CI never runs these gtest binaries via ctest (it uses nodejs_test_harness), so make build now uses DISCOVERY_MODE PRE_TEST, deferring discovery off the build path. Local cmake keeps POST_BUILD (the dedupe custom command still applies, and already no-ops on a missing manifest).

3. gatsby segfault flake (v8-linux framework apps).
gatsby build on the Node.js reference intermittently segfaults in the lmdb native addon. Retry a build up to 3×, but only when the log shows a native crash (SIGSEGV/SIGABRT/SIGBUS/core dumped/worker-signal). A deterministic non-zero build error never matches and is never retried.

4. gatsby hang (v8-macos framework apps).
gatsby build deterministically hangs on macOS runners (gatsby-worker/sharp) and the harness has no build-step timeout, so a retry cannot help. Exclude js-gatsby-* on macOS only, mirroring quickjs-macos. Linux keeps building gatsby with the retry above.

Validation

• build: include simdjson library #1 proven end-to-end locally: with the fix the runner resolves to build-edge/edge (the binary the native job actually builds). The bug never reproduces locally only because the QuickJS binary happens to be present; CI doesn't build it.
• libuv: provide stubs #3 logic unit-tested: retries on crash signatures, does not retry on ELIFECYCLE / Cannot find module / render errors.
• Cannot run a simple Node.js app on native #2 / Refactor 1 #4 are macOS-runner-only and verified in this PR's CI run.

Caveat on #3: the retry re-runs gatsby build in place (dirty .cache) rather than a fresh checkout; gatsby's incremental build tolerates this. If it proves insufficient, cleaning artifacts between retries is a follow-up.

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