stream: limit iter from sync iterable batches

[trivikr]

Description

This updates stream/iter normalization to use bounded batches for sync
iterable input.

from() no longer batches sync iterables into one unbounded batch. Instead, it
flushes at FROM_BATCH_SIZE, improving first-batch latency and bounding peak
batch memory. fromSync() now also coalesces plain Uint8Array values from
sync iterables into bounded batches, allowing pipeToSync() to use writevSync
for those sources.

Benchmarks

Results showed statistically significant first-batch latency improvements for
from(syncIterable):

• +3.19% for 256 chunks
• +5.01% for 4096 chunks
• +2.80% for 16384 chunks

The benchmark also showed a +6.91% writevSync improvement for the 256 chunk
case. The 4096 chunk writev result was not statistically significant.

                                                                                         confidence improvement accuracy (*)   (**)  (***)
streams/iter-from-batching.js n=100 chunkSize=16 chunks=256 method='from-sync-writev'           ***      6.91 %       ±1.99% ±2.65% ±3.46%
streams/iter-from-batching.js n=100 chunkSize=16 chunks=4096 method='from-sync-writev'                  -2.63 %       ±3.60% ±4.84% ±6.41%
streams/iter-from-batching.js n=1000 chunkSize=16 chunks=16384 method='from-first-batch'        ***      2.80 %       ±1.39% ±1.86% ±2.44%
streams/iter-from-batching.js n=1000 chunkSize=16 chunks=256 method='from-first-batch'          ***      3.19 %       ±1.27% ±1.68% ±2.19%
streams/iter-from-batching.js n=1000 chunkSize=16 chunks=4096 method='from-first-batch'         ***      5.01 %       ±2.60% ±3.48% ±4.59%

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/performance
• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 57.89474% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.02%. Comparing base (2edd842) to head (d08b083).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/from.js	57.89%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63324      +/-   ##
==========================================
- Coverage   90.04%   90.02%   -0.03%     
==========================================
  Files         714      714              
  Lines      225338   225377      +39     
  Branches    42598    42610      +12     
==========================================
- Hits       202897   202886      -11     
- Misses      14236    14267      +31     
- Partials     8205     8224      +19     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/from.js	86.05% <57.89%> (-2.77%)	⬇️

... and 29 files with indirect coverage changes

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