feat: identify completed build output

[peter-jerry-ye]

Why

Clients that capture output from multiple concurrently executed builds need to attribute each output chunk to the build that produced it. The existing progress callback exposes only the output text, so that provenance is lost at the n2 boundary.

What changed

• expose a BuildOutputCallback receiving both BuildId and captured output
• add new_with_build_output constructors for dumb and fancy console progress
• preserve the existing new constructors by adapting their text-only callbacks
• cover the callback identity contract with a focused regression test

Why this is correct and minimal

Progress::task_finished already receives the authoritative BuildId together with the matching TaskResult, so the callback forwards existing information without reconstructing identity. Execution, scheduling, capture, and default console behavior are unchanged, and existing callers remain source-compatible.