Move STDIO input reads off the cooperative executor

[lynnswap]

Purpose

Fix the main-branch CI hang observed in run 29402610133.

The apparent progress-test stall was buffered output. Under a single cooperative worker, FileHandle.AsyncBytes performed a blocking stdin read and starved the task responsible for writing and closing the adapter pipe.

Changes

• move STDIO input ownership to a dedicated DispatchIO channel
• bound read-ahead to one 64 KiB operation while preserving input order and backpressure
• close only the duplicated input descriptor and preserve the caller-owned FileHandle
• preserve graceful EOF draining, immediate read-error aborts, cancellation, and descriptor-cleanup completion
• add event-driven tests for bounded reads, exact-boundary EOF, active-read cancellation, read failures, deinitialization, and caller ownership
• use an injected clock to prove read failures do not enter the EOF drain loop
• bound the test event observation so regressions fail instead of hanging CI; elapsed time is not used to establish behavior

Testing

• ./scripts/check.sh
  • 956 package tests passed
  • 24 process runtime tests passed
  • 13 STDIO adapter tests passed
• strict single-worker reproduction passed with LIBDISPATCH_COOPERATIVE_POOL_STRICT=1 (14 focused tests)
• branch-wide codex-review against pinned main@d2b7e76d: no findings

[lynnswap]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 64ad7593f0

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[lynnswap]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

Reviewed commit: 3b41ed25e9

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".