test(parity): runtime lane paralelismo estable por default (gate determinista)

-parallel 4 sobrecargaba el daemon docker en runners contendidos y hacía flakear
casos compose/pesados transitoriamente (matchean aislados). El gate (task
parity:runtime) ahora default -parallel 2, override via PARITY_PARALLEL. Quita el
caveat de la corrida limpia.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: aledbf

Taskfile.yml

@@ -95,7 +95,10 @@ tasks:
       - sh: docker info >/dev/null 2>&1
         msg: Docker is required for runtime parity
     cmds:
-      - PARITY_REPORT_FILE={{.ROOT_DIR}}/artifacts/parity-runtime.json PARITY_STRICT=true PARITY_LANE=all go test ./internal/cli -run TestParityMatrix -count=1 -parallel 4 -timeout 30m -v {{.CLI_ARGS}}
+      # -parallel default 2 for gate determinism: higher values overload the docker
+      # daemon on contended runners and transiently fail compose/heavy cases (they
+      # match in isolation). Raise via PARITY_PARALLEL on beefy/idle runners.
+      - PARITY_REPORT_FILE={{.ROOT_DIR}}/artifacts/parity-runtime.json PARITY_STRICT=true PARITY_LANE=all go test ./internal/cli -run TestParityMatrix -count=1 -parallel {{.PARITY_PARALLEL | default "2"}} -timeout 40m -v {{.CLI_ARGS}}
       - go test ./internal/cli -run TestPublishParity -count=1 -timeout 10m -v
 
   clean:

internal/cli/parity_matrix_test.go

@@ -9,10 +9,12 @@
 //
 // Recommended invocation for the full runtime lane (docker required):
 //
-//	PARITY_LANE=all go test ./internal/cli -run TestParityMatrix -parallel 6 -timeout 30m
+//	PARITY_LANE=all go test ./internal/cli -run TestParityMatrix -parallel 2 -timeout 40m
 //
-// ~5.5 min vs ~26 min sequential. Higher -parallel values increase docker
-// contention and transient flakes; 6 is a stable default on an 8-core host.
+// Higher -parallel is faster but overloads the docker daemon on contended runners
+// and transiently fails compose/heavy cases (which match in isolation), so the gate
+// (task parity:runtime) defaults to -parallel 2 for determinism; raise via
+// PARITY_PARALLEL on idle/beefy runners.
 package cli
 
 import (