Summary

Implement the reporter described in #344: a typed LogEvent enum mirroring
@pnpm/core-loggers, a trait Reporter per the #339 capability shape, an
NdjsonReporter that writes pnpm's wire format to stderr, a
SilentReporter for --reporter=silent, generic threading through the
install entry point, and the spawn-and-pipe wiring that feeds pacquet's
NDJSON into @pnpm/cli.default-reporter for default rendering.

This is the engine only. Backfilling missing log events in existing crates
and documenting the porting convention land in separate issues (linked
below).

Scope

• New crate (likely crates/reporter, or extend an existing one if a
  clearer home emerges):
  • LogEvent enum with one variant per pnpm log channel from
    @pnpm/core-loggers.
    Serde attributes produce the bunyan envelope (level, time, name)
    plus channel-specific payload, matching what pnpm emits under
    --reporter=ndjson.
  • trait Reporter { fn emit(event: &LogEvent); } per Refactor and document the dependency injection pattern for tests #339 (associated
    function, no &self).
  • NdjsonReporter unit struct:
    • Module-level static OnceLock<Sender<Vec<u8>>> for the writer-task
      MPSC sender.
    • Module-level static throttle map for pnpm:fetching-progress
      (200ms per package, matching pnpm's reporter-side throttle).
    • Thread-local Vec<u8> serialization buffer per emit.
    • One dedicated writer task draining the channel and write_all-ing to
      stderr in batches.
  • SilentReporter unit struct: no-op emit.
• --reporter=default|ndjson|silent CLI flag in pacquet-cli. The entry
  point dispatches to install::<NdjsonReporter> or
  install::<SilentReporter>; default and ndjson share the same sink,
  default additionally spawns the JS reporter and pipes stderr into it.
• Spawn-and-pipe wiring for --reporter=default: spawn
  @pnpm/cli.default-reporter (or pnpm --reporter=default in consumer
  mode — TBD) as a child process and pipe pacquet's stderr to its stdin.
  The PR picks one of the three delivery options from Use pnpm's @pnpm/cli.default-reporter for terminal output (via NDJSON) #344 and documents
  the choice.
• Identifier interning: Arc<str> (or lasso / ustr) for package
  names/versions threaded through events. Hooked at the resolver boundary
  so call sites already hold cheap clones.
• One real channel wired end-to-end as a smoke test (e.g. pnpm:stage for
  "importing"), verified to render identically to pnpm.
• Schema pinning: pacquet's Cargo.toml records the pnpm version it
  targets; add a CI check that emitted JSON parses against the
  corresponding upstream @pnpm/core-loggers shapes (TS-side script or a
  serde round-trip into our own duplicate of those types).

Out of scope (tracked separately)

• Wiring all existing pacquet call sites to emit events.
• AGENTS.md / CODE_STYLE_GUIDE.md guidance for porting log emissions from
  pnpm.
• Native Rust reporter — explicitly deferred per Use pnpm's @pnpm/cli.default-reporter for terminal output (via NDJSON) #344.

Acceptance criteria

• pacquet install --reporter=ndjson emits NDJSON whose lines parse cleanly
  when piped into pnpm --reporter=default (or
  npx @pnpm/cli.default-reporter), with at least one channel rendering
  identically to pnpm.
• pacquet install --reporter=silent produces no output beyond what
  --reporter=silent produces in pnpm.
• pacquet install --reporter=default spawns the JS reporter and renders
  pnpm-shaped output for the wired channel.
• just ready is green; the recording-fake pattern from Refactor and document the dependency injection pattern for tests #339 is used at
  least once.

Parent

Design: #344. Roadmap line item: pnpm/pnpm#11633.