Expose `--verbose` and `--threads` flags through `aztec compile` to `bb aztec_process`

[AztecBot]

Context

When users run aztec compile and it hangs during VK generation, there's no way to get diagnostic output without manually running bb aztec_process --verbose as a separate step. Similarly, there's no CLI flag to control thread count — users must set the HARDWARE_CONCURRENCY env var.

This came up from the Dark Forest team hitting a hang during compilation of a large contract. The only workaround today is:

1. Run nargo compile separately
2. Run bb aztec_process --verbose -i <artifact> manually
3. Set export HARDWARE_CONCURRENCY=N to control threads

Or use export BB_VERBOSE=1 which the spawn inherits — but this is undiscoverable.

What's needed

1. Pass --verbose through aztec compile

bb aztec_process already supports --verbose (defined in cli.cpp:628), but aztec compile (compile.ts:153) hardcodes only -i args:

const bbArgs = artifacts.flatMap(a => ['-i', a]);
await run(bb, ['aztec_process', ...bbArgs]);

Add a --verbose flag to aztec compile that forwards --verbose to bb aztec_process.

2. Add --threads N flag to bb

Currently thread count is controlled only via the HARDWARE_CONCURRENCY env var (thread.cpp:14). Add a --threads N CLI option to bb (at least for aztec_process) that sets thread concurrency, then expose it through aztec compile --threads N.

Relevant files

• yarn-project/aztec/src/cli/cmds/compile.ts — aztec compile command definition and bb invocation
• barretenberg/cpp/src/barretenberg/bb/cli.cpp — bb CLI, aztec_process subcommand (line 607+)
• barretenberg/cpp/src/barretenberg/common/thread.cpp — HARDWARE_CONCURRENCY env var handling
• barretenberg/cpp/src/barretenberg/api/aztec_process.cpp — artifact processing with parallel VK generation