Fix stdout buffering to enable streaming output

[prateek]

Fixes #481 (Request: streaming output).

Problem

When xcbeautify is piped to another process (the typical usage: xcodebuild ... | xcbeautify | ...), the C runtime defaults to full buffering for stdout. Since the CLI used print() for all output, lines accumulated in a ~4 KB buffer before being flushed to the downstream consumer. This meant CI agents, log watchers, or other tools couldn't see results as they were produced; they had to wait for the buffer to fill or for the process to exit.

Solution

Add setvbuf(stdout, nil, _IONBF, 0) at process start to disable C stdio buffering on stdout. This is a single-line fix that makes all existing print() calls flush immediately, with no other code changes needed.

Uses @preconcurrency import Foundation to satisfy Swift 6 strict concurrency for the global stdout symbol.

Why setvbuf over the alternatives

Approach	Tradeoff
setvbuf(stdout, nil, _IONBF, 0) (chosen)	One line, uses standard print(), single I/O layer, no error handling needed
POSIX write(STDOUT_FILENO, ...)	Bypasses stdio entirely but mixes two I/O layers on the same fd, requires manual partial-write and EINTR handling
fflush(stdout) after each print	Requires touching every output site; easy to miss one

Changes

• Sources/xcbeautify/Xcbeautify.swift: add setvbuf(stdout, nil, _IONBF, 0) at the top of run() and use @preconcurrency import Foundation
• tools/test-streaming: add a CLI streaming test using FIFO-to-FIFO reads with read -t to verify each line flushes immediately
• tools/cli-tests: integrate the streaming test into the existing CLI suite

Testing

• swift test
• swift build --configuration release -Xswiftc -warnings-as-errors
• ./tools/lint
• ./tools/cli-tests

[Copilot]

Pull request overview

This PR addresses issue #481 by ensuring xcbeautify streams output immediately when its stdout is piped, rather than being delayed by C stdio full-buffering.

Changes:

• Disable stdout buffering at CLI startup via setvbuf(stdout, nil, _IONBF, 0).
• Add a FIFO-based streaming regression test (tools/test-streaming) and run it from the existing CLI test harness.
• Add environment/setup notes to AGENTS.md (Cursor Cloud-specific).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
Sources/xcbeautify/Xcbeautify.swift	Disables stdout buffering at process start (and adjusts imports for Swift 6 concurrency).
tools/test-streaming	Adds an integration test to validate per-line flushing through pipes using FIFOs + timeouts.
tools/cli-tests	Integrates the streaming test into the CLI test suite.
AGENTS.md	Adds Cursor Cloud environment/tooling notes and CLI testing guidance.

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[prateek]

@copilot apply changes based on this feedback

[prateek]

@copilot apply changes based on this feedback