fix(ci): raise the unit-shard heap ceiling to 8192 MB to stop the SIGABRT OOM

The 8 unit shards run under V8 coverage instrumentation, which retains far more
memory than the bare suite. With the 4096 MB ceiling they began aborting with
exit 134 ("Ineffective mark-compacts near heap limit") at ~4086 MB as the
provider catalogue grew during the v3.8.50 cycle: every test in the shard passed
and the process died at the end, which reads as a test failure without being one.

Aligns test:unit:ci:shard and test:unit:serial with the 8192 MB the non-sharded
variants already use. GitHub-hosted runners have 16 GB, so the headroom is real.
Validated by the CI run on this commit — the shards are the gate.

Refs #10692

.github/workflows/ci.yml

@@ -792,6 +792,14 @@ jobs:
       # D3 (plano mestre): a coverage é coletada NESTE mesmo run (c8/NODE_V8_COVERAGE propaga
       # aos filhos através do npm) — elimina a matrix Coverage Shard ×8, que re-executava a
       # suíte inteira só para medir o gate. Padrão usado pelo CI do próprio nodejs/node.
+      # Heap: os shards rodam sob instrumentacao de cobertura do V8, que retem muito
+      # mais memoria que a suite crua. Com o teto antigo de 4096 MB os shards passaram
+      # a abortar com SIGABRT (exit 134, "Ineffective mark-compacts near heap limit")
+      # ao redor de 4086 MB conforme o catalogo de providers cresceu no ciclo v3.8.50 —
+      # todos os testes passavam e o processo morria no fim, o que le como falha de
+      # teste sem ser. O teto vive em `test:unit:ci:shard` (package.json) e agora
+      # acompanha os 8192 MB ja usados pelas variantes nao-shardadas; os runners
+      # GitHub-hosted tem 16 GB.
       - name: Unit tests (shard ${{ matrix.shard }}/8) with V8 coverage
         env:
           TEST_SHARD: ${{ matrix.shard }}/8