stream: skip unobserved 'readable' emission at EOF

[mcollina]

When a stream reaches its end via push(null) while nobody is observing it — no 'readable' listener, not flowing, no pending readable need — the deferred 'readable' emission fires into the void one tick later, costing a nextTick and a dead emit per stream. This happens for every HTTP request whose body the handler never reads (every body-less request).

This change skips scheduling in that case and records that the end-of-stream notification is owed (a new readable-state bit). Attaching a 'readable' listener later redeems it — the listener receives 'readable' and then 'end' in the usual order — and read()/resume()/'data' reach the end of the stream on their own, as they already did. A consuming read() clears the owed notification.

One deliberate behavior change, covered by the added test: a 'readable' listener attached more than a tick after an unobserved end now receives the owed 'readable' before 'end', where it previously received only 'end' (the emission it would have needed had already fired with no listeners attached). This matches the documented behavior that 'readable' is also emitted once the end of the stream is reached.

For a hello-world HTTP server this removes one nextTick (7 → 6 per request on main) and one dead emit (9 → 8) per request. Complements the teardown work in #65732; combined they bring the request lifecycle to 4 ticks/request.

benchmark/compare.js --runs 12 --filter readable streams shows no statistically significant change across all 14 benchmarks. test/parallel/test-stream*, test-http*, test-https*, test-net-*, test-tls-*, test-zlib*, fs stream, readline and child_process suites all pass (1722 tests).

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AI assisted, humanly reviewed

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.99%. Comparing base (7b0de5e) to head (70d380e).
⚠️ Report is 81 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65749      +/-   ##
==========================================
- Coverage   90.05%   89.99%   -0.06%     
==========================================
  Files         754      757       +3     
  Lines      255722   257758    +2036     
  Branches    48314    48889     +575     
==========================================
+ Hits       230281   231971    +1690     
- Misses      16555    16856     +301     
- Partials     8886     8931      +45     

Files with missing lines	Coverage Δ
lib/internal/streams/readable.js	96.59% <100.00%> (+0.03%)	⬆️

... and 93 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/76910/