Fix unhandled rejection from process.stdin.close() in execProcess (#14445)

[gordonwoodhull]

Summary

Fixes #14445 — intermittent Uncaught (in promise) TypeError: Writable stream is closed or errored. aborting renders on Linux.

src/core/process.ts:97 called process.stdin.close() without awaiting the returned Promise. If the child closes/errors its stdin before the parent's close completes, the Promise rejects with Writable stream is closed or errored. Because it was fire-and-forget, the rejection escaped execProcess's try/catch and surfaced on a later microtask as an uncaught Deno rejection — bypassing the try/catch in analyzeNeededPackages that was meant to fall back gracefully when typst-gather analyze fails.

Reproduced on Ubuntu CI at roughly a 1% race rate via the child exits without reading stdin scenario in the regression test (commit 48199c3, bucket 11 job log). Not reproduced on macOS arm64.

Changes

• src/core/process.ts — await process.stdin.close() inside a try/catch. The child's exit status already reflects any real failure, so the close rejection is not actionable from execProcess.
• src/command/render/output-typst.ts — surface the captured stderr in the fallback warning from analyzeNeededPackages, so the next user with a broken typst-gather binary can diagnose without objdump.
• tests/unit/exec-process-stdin.test.ts — regression test running four execProcess scenarios 1000 iterations each with a global unhandledrejection listener that asserts no rejection fires. The first scenario (child exits without reading) is the one that reproduced on Ubuntu pre-fix.

Test plan

• Unit tests pass on macOS arm64 (./run-fast-tests.sh unit/exec-process-stdin.test.ts, 31s, 4 passed)
• Regression test reproduced the bug on Ubuntu CI before the fix (2/200 unhandled rejections in scenario 1)
• Same regression test passes on Ubuntu CI after the fix