🐛 Settle join() only after output clears Stdio middleware

[taras]

Motivation

Fixes #244. On POSIX, join() and expect() read the useNativeProcess result directly, which settles on the child-process close event — while the Stdio middleware tasks may still be forwarding the final chunks. A caller could observe the exit status before all output had passed through its middleware and the public signals: finalize a decoder early, persist truncated output, or dismantle a middleware scope that still owns pending chunks. The win32 adapter already drained before resolving. Separately, a throwing Stdio handler crashed the host scope on posix and could deadlock the drain wait on win32.

Approach

Stacked on #247; supersedes #245, whose fix and regression tests are carried here. No changes to native.ts — the resource's close-settled result is correct; this is a middleware-layer change in the adapters:

• posix gains the same drain-then-resolve task win32 already had: processResult settles only after the resource result plus both middleware tasks completing, so a blocked handler keeps join() pending. The two adapters are now symmetric. join() stays close-settled per process: no way to observe exit — join() settles on close (exit + stdio EOF), which can outlive the command #228's decision — the drain adds to the close wait, never replaces it — and Exec.join()/Exec.expect() inherit the guarantee.
• On both platforms, a failing Stdio handler resolves processResult with Err, so join()/expect() throw the handler's error instead of hanging, and the middleware tasks always close their signals and resolve their done-resolvers in a synchronous finally, so teardown can never wait on a dead task.
• Regression tests gate every chunk in middleware and release the gate only once the internal CloseEvent seam (src/exec/internal.ts, not exported from mod.ts) confirms the close event was received — join() therefore settles strictly after close yet must still show complete output, with no scheduler sleeps. Mutation-verified: deleting the posix drain fails both completeness tests deterministically. If a public exited() lands later (process: no way to observe exit — join() settles on close (exit + stdio EOF), which can outlive the command #228's original ask), the seam is deleted and the tests gate on that instead.

40/40 process tests × 3 runs on top of #247; typecheck/lint/format clean. The platform test matrix runs once this retargets to main after #247 merges.

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commit: f2964df