fix: honor per-target level filter on single-element targets array

[mcollina]

Summary

• Fixes level filter not work for transports #1996 — the per-target level filter was silently ignored when pino.transport({ targets: [...] }) contained a single element.
• The single-target shortcut in lib/worker.js returned the raw stream directly, bypassing pino.multistream (which is the only place that consults targets[i].level).
• Now, when level is set on the single target/pipeline, we fall through to the multistream path that already enforces per-target filtering. When level is not set, behavior is unchanged.

Root cause

lib/worker.js:126:

if (targetStreams.length === 1) {
  return targetStreams[0].stream
}

The level captured at line 85 (level: t.level) was collected and then discarded for the one-element case. So targets: [{ level: 'error', target: '...' }] emitted info/warn/debug logs anyway — the exact symptom reported in #1996.

Fix

lib/worker.js:

if (targetStreams.length === 1 && targetStreams[0].level == null) {
  return targetStreams[0].stream
}

lib/transport.js — extend usesMultistream so transportUsesMultistreamSym is set whenever per-target filtering will run in the worker. This keeps the existing formatters.level warning (pino.js:169) consistent: if filtering happens, the numeric level invariant must hold.

Scope

Intentionally minimal — fixes the case where level is set (the #1996 reproducer) without altering the no-level case. The remaining asymmetry (single target with no level inherits logger.level instead of defaulting to info like multistream does) is a separate semver-major decision and is left for a follow-up.

Test plan

• New TDD test reproducing level filter not work for transports #1996 verbatim (targets: [{ level: 'error', ... }] → only error reaches destination)
• New regression guard for the no-level case (single target with no per-target level keeps logger.level gating)
• test/transport/core.test.js (38 tests) — all pass
• test/transport/{pipeline,targets,sync-true,sync-false,crash,uses-pino-config}.test.js — all pass
• test/transport-stream.test.js, test/multistream.test.js — all pass
• test/levels.test.js, test/is-level-enabled.test.js — all pass
• npm run lint — clean

Closes #1996

[jsumners]

CI is failing.