fix(meteor-promise): create pool fibers without inherited async context

[jbaczuk-qualia]

Problem

meteor-promise runs every Promise.asyncApply / fibered .then callback on a fiber from a shared pool. Between jobs each pool fiber parks itself with Fiber.yield() in its own async scope. The fibers fork gives every fiber an AsyncResource whose AsyncLocalStorage stores are copied from the creator once, so a pool fiber created while method M was running carries M's OpenTelemetry context forever, and every idle yield it ever makes is attributed to M.

On qualia-impacttitle this produced ~11k/s of meteor_fiber_yields_total labelled refreshDocs, IncomingWires.OrderMatches.refresh, etc. while none of those methods were running (the process had created 205 fibers in 12 hours; all of the traffic was the customer's GraphQL poller going through the pool).

Fix

• Create pool fibers inside a module-load-time AsyncResource (PRISTINE_ASYNC_SCOPE) so they inherit no stores at all.
• Set fiber._meteorPromisePoolIdle around the idle Fiber.yield() so Fiber.yield instrumentation can label the park explicitly (qualia's prom-client will count it as fiber_pool/idle; companion PR in the qualia repo).

Job callbacks still run inside their per-job AsyncResource (entry._ar) and see the submitter's context as before. Meteor environment variables were already unaffected: fibers_async.js resets _meteor_dynamics on fiber start, and jobs read them through the job scope.

Test

test/tests.js › "fiber pool async context": grows the pool under ALS store creator, then runs 50 trivial jobs under user and asserts no yield sees creator, jobs see user, and the flagged idle yields see no store. Fails against the previous fiber_pool.js with a yield during the user phase saw the creator's store; passes with this change. Runs only on the native-Promise pass (the promise polyfill's asap scheduler batches unrelated reactions under one async context).

Verified on Node 18.16.1 (same as prod).

Versions

meteor-promise 0.9.1-2 → 0.9.1-3, promise package 0.12.2 → 0.12.3 (Npm.depends updated).

🤖 Generated with Claude Code

[jbaczuk-qualia]

flag this fiber as a idle pool fiber before yielding so we can detect it in the app