refactor(gui): extract pure helpers from MainWindow.cpp (#3351 Phase 0)

[ten9876]

Summary

First PR of the #3351 monolith-decomposition series. Pure code motion — no behavior change.

Extracts the ~20 file-scope static helpers that have no MainWindow dependency (no members, no mutable file-scope state) from MainWindow.cpp into a new MainWindowHelpers.{h,cpp}:

Group	Functions
Network diagnostics tooltip	formatNetworkMs, formatNetworkSeqErrors, buildNetworkTooltip
TNF tooltip	tnfFrequencyHz, formatTnfFrequency, formatTnfDepth, buildTnfTooltip
Memory/passive spot-ID math	memorySpotId, memoryIndexFromSpotId, isPassiveLocalSpotId, memorySpotLabel, memorySpotComment + the two ID-base constants (used only by these)
Client connection parsing	splitClientField, parseClientHandle, buildDisconnectClients ×3, cleanClientDisplayText, clientConnectionStatusMessage
Misc	buildBandStackIndicatorPixmap, shortcutSequenceFromKeyEvent, macDaxDriverInstalled

Deliberately left behind: the shortcut-lease helpers (textInputCaptured, shortcutGuard, leaseHolderBusy, …) — they read the mutable statics s_keyboardShortcutsEnabled / s_sliderShortcutLeaseActive and are destined for the Shortcuts TU in Phase 1. The new header documents the rule: state-coupled helpers don't belong in this file.

Notes

• Functions lose internal linkage (static → namespace-scope external). All live in namespace AetherSDR; no ODR risk.
• The long-standing Linux -Wunused-function: macDaxDriverInstalled warning at the tail of every build disappears — its only caller is inside #ifdef Q_OS_MAC, and external-linkage functions don't trigger that warning.
• One transplant fix: the CategoryStats convenience overload of formatNetworkSeqErrors became formatCategorySeqErrors (anon-namespace, private to the new TU). As an anonymous-namespace overload of the same name it would have hidden the namespace-scope two-int overload from unqualified lookup inside the TU.

Numbers

• MainWindow.cpp: 19,474 → 19,138 lines
• New files: MainWindowHelpers.h (90), MainWindowHelpers.cpp (379)

Verification

• Full build clean (Linux x86, RelWithDebInfo)
• All 32 ctest suites pass
• Diff is reviewable as: new files = verbatim copies of the deleted statics (modulo the one rename above); MainWindow.cpp = deletions + one #include

Next in the series (separate PRs): Phase 1a MainWindow_Controllers.cpp, then the remaining per-seam TUs per the plan on #3351.

Refs #3351

[aethersdr-agent]

Hi @ten9876, thanks for the PR — and don't worry, this CI failure is not caused by your changes. 🎉

What failed

On commit b48c3aa, the check-windows job (run 27290587519) failed at the "Setup hidapi (HID encoder support)" step — an environment-provisioning step that runs before your code is ever configured or compiled. The Configure and Build steps were skipped as a result, so the failure tells us nothing about the code in this PR.

That step runs scripts/setup/setup-hidapi.ps1, which downloads the hidapi 0.14.0 source tarball from GitHub and builds it with MSVC on the runner (see .github/workflows/ci.yml, the step right after "Setup MSVC environment"). Failures here are almost always transient — a flaky download from the GitHub release CDN or a hiccup on the Windows runner — and unlike windows-installer.yml, the check-windows job doesn't cache the hidapi build, so it re-downloads and rebuilds it on every run.

Why we're confident it's infrastructure, not your code

• Your PR touches CMakeLists.txt, src/gui/MainWindow.cpp, and the new src/gui/MainWindowHelpers.{h,cpp} — nothing related to CI workflows, setup scripts, or hidapi.
• Every check that actually compiled your code passed: check-macos ✅, build ✅, analyze (cpp) ✅, CodeQL ✅, Qt Accessibility Static Analysis ✅.
• The Windows job died before reaching the Configure step, so your code was never even handed to the compiler there.

How to fix it

Just re-run the failed job — no code changes needed. If you don't have re-run permissions on this repo, pushing an empty commit works too:

git commit --allow-empty -m "ci: retrigger check-windows" && git push

or a maintainer can re-run it from the Actions page.

Thanks for taking on the MainWindow decomposition work (#3351 Phase 0) — extracting those stateless helpers into their own TU is exactly the right first slice, and the header comment documenting the "no MainWindow state" rule for future additions is a nice touch. 👍

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_{🤖 aethersdr-agent · cost: $7.5456 · model: claude-fable-5}

[jensenpat]

Reviewed line-by-line: pure code motion as advertised — new files are verbatim copies of the deleted statics, constants preserved, the formatCategorySeqErrors rename is correct and applied at all call sites. All CI green.