Allow batch executors to share the parallel pool with discrete tasks

[jgannonjr]

• I'd be willing to implement this feature (contributing guide)

Description

Batch executors (batchImplementation / preferBatch) currently monopolize the task orchestrator: while a batch is running, no discrete tasks start, even when --parallel / nx.json parallel has spare capacity.

Add an opt-in executor setting (e.g. batchParallelismCost) so a batch counts as N parallel slots (typically 1) instead of blocking the coordinator. Discrete tasks whose dependencies are satisfied can then run in the remaining capacity.

Motivation

In mixed monorepos (e.g. TypeScript + Cargo), preferBatch: true is valuable for coalescing many crate tasks into one cargo check -p a -p b -p c, but it serializes the entire run-many / affected pipeline behind that batch. TypeScript typecheck / lint / build sit Waiting until Cargo finishes.

Today the tradeoff is:

1. Keep batching → starve non-batch tasks, or
2. Disable preferBatch → lose multi--p coalescing

Core support for “batch occupies 1 parallel slot” fixes that for any batch executor (Cargo plugins, Gradle/Maven if they opt in, custom batch executors)—without changing default batch behavior.

Suggested Implementation

On executor registration (executors.json / ExecutorConfig):

{
  "implementation": "./executor",
  "batchImplementation": "./batch",
  "preferBatch": true,
  "batchParallelismCost": 1
}