Fix Linux build: replace fputs(_:stderr) with FileHandle.standardError

[leogdion]

The error

The Examples workflow's Test MistDemo on Ubuntu job fails to compile:

Examples/MistDemo/Sources/MistDemo/MistDemo.swift:44:33: error: reference to var 'stderr'
is not concurrency-safe because it involves shared mutable state
44 | fputs("❌ \(message)\n", stderr)
| `- error

Why it is Linux-only

glibc declares the stream as extern FILE *stderr — a plain mutable global — which Swift imports as a global var. Under Swift 6 strict concurrency, referencing a non-Sendable, non-isolated mutable global is an error.

Darwin's headers expose stderr differently (via __stderrp), and the Darwin overlay imports it in a form the concurrency checker accepts, so the exact same line compiles cleanly on macOS. That's why this got through review and the macOS jobs: it cannot reproduce locally on a Mac.

The line was introduced in #429 (0b6bea9).

The fix

One-line change in Examples/MistDemo/Sources/MistDemo/MistDemo.swift: write through FileHandle instead of the C stream.

// Glibc exposes `stderr` as a mutable global, which Swift 6 strict
// concurrency rejects; FileHandle keeps this portable to Linux.
FileHandle.standardError.write(Data("❌ \(message)\n".utf8))

This is the idiom already used everywhere else in this codebase for stderr output — MistDemoConfig+DatabaseConfiguration.swift, LookupCommand.swift, LookupAllRecordsCommand.swift, ModifyCommand.swift, ModifyZonesCommand.swift, DiscoverAllUserIdentitiesCommand.swift, and BushelCloudKit's ConsoleOutput — so the fix matches existing style rather than introducing a new pattern. internal import Foundation was already present in the file, so no import change was needed (the repo's explicit-access-modifier import convention is preserved).

I grepped all of Examples/ and Sources/ for stderr/stdout: this was the only code reference. Every other hit is a doc comment or a test asserting on help text, none of which touch the C globals. No other changes are included in this PR.

Verification

Check	Result
swift build in Examples/MistDemo (macOS, Swift 6.3)	pass
swift test in Examples/MistDemo (macOS)	pass — 994 tests / 295 suites, 1 known issue
swift build + swift test at repo root (macOS)	pass — 618 tests / 195 suites
swiftlint --strict + swift-format lint --strict on the changed file	clean
Linuxswift build — docker run --rm swift:6.3 in Examples/MistDemo (same image the CI job uses)	pass — "Build complete!", the error is gone
Linuxswift test — same container	pass — 994 tests / 295 suites, 1 known issue (matches the macOS run)

Linux verification was actually performed, not assumed: the pre-fix error reproduces only on Linux, and the swift:6.3 container build now completes with exit code 0.

Note on LINT_MODE=STRICT ./Scripts/lint.sh: it reports 54 pre-existing violations on this base branch (file-name, type-contents-order, line-length, etc. across Sources/, Tests/, and Scripts/OpenAPITools/Package.swift). None are in the file this PR touches, and the count is unchanged by this PR — Scripts/lint.sh doesn't even scan Examples/. Worth flagging separately: when $CI is unset, lint.sh runs swift-format --in-place and swiftlint --fix regardless of LINT_MODE, so running it locally rewrites unrelated files (and the locally-resolved swift-format disagrees with the pinned one). That's out of scope here.

🤖 Generated with Claude Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.39%. Comparing base (61235b5) to head (ef84566).

Additional details and impacted files

@@ Coverage Diff @@## v1.0.0-beta.4 #441 +/- ##
=================================================
- Coverage 81.45% 81.39% -0.07% 
=================================================
Files 191 191 Lines 4719 4719 =================================================
- Hits 3844 3841 -3 - Misses 875 878 +3 

Flag	Coverage Δ
mistdemo-spm-macos	11.31% <ø> (ø)
mistdemo-swift-6.2-jammy	11.31% <ø> (?)
mistdemo-swift-6.2-noble	11.31% <ø> (?)
mistdemo-swift-6.3-jammy	11.31% <ø> (?)
mistdemo-swift-6.3-noble	11.31% <ø> (?)
mistdemo-swift-6.4-jammy	11.31% <ø> (?)
mistdemo-swift-6.4-noble	11.31% <ø> (?)
spm	80.01% <ø> (+0.02%)	⬆️
swift-6.1-jammy	80.11% <ø> (-0.15%)	⬇️
swift-6.1-noble	80.13% <ø> (-0.07%)	⬇️
swift-6.2-jammy	80.01% <ø> (-0.32%)	⬇️
swift-6.2-noble	80.13% <ø> (+0.14%)	⬆️
swift-6.3-jammy	80.16% <ø> (+0.14%)	⬆️
swift-6.3-noble	79.97% <ø> (-0.07%)	⬇️
swift-6.4-jammy	80.01% <ø> (-0.24%)	⬇️
swift-6.4-noble	80.13% <ø> (-0.03%)	⬇️

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