feat(linux): the Go half of OpsLog builds for Linux

Measured rather than guessed: the whole repository was cross-compiled for
linux/amd64 and the gaps closed one by one. There were fewer than expected.

Flex and TCI were never Windows-specific — they carried //go:build windows by
inheritance and import nothing but net and gorilla/websocket. Untagged, no code
change. The two backends a Linux operator is most likely to own were already
portable.

Audio was 560 lines, not 2287: only devices.go and engine.go touch WASAPI, while
manager.go, recorder.go, wav.go and mp3.go were pure Go wearing the tag by
association. The whole platform surface is seven functions, now implemented a
second time on PulseAudio through github.com/jfreymuth/pulse — pure Go over the
server socket, so the no-cgo rule survives, and PipeWire answers the same
protocol. The fixed 16 kHz mono format and the server-side resampling mirror
what AUTOCONVERTPCM does on Windows, for the same reason.

OmniRig is the only real loss, and its backend still EXISTS off Windows rather
than being compiled out of app.go: a settings database is portable, so an
operator moving a profile across keeps "omnirig" saved and must be told to pick
a native backend instead of meeting a nil one.

The parts where Linux is not Windows, and where a compile-only stub would have
been a silent bug:

  - data dir: still beside the binary, but ~/.local/share/OpsLog/data when that
    folder belongs to the system — decided by trying the write, because /opt and
    /usr/local are writable on some stations and not others.
  - single instance: an flock, not a pid file. The kernel drops it however the
    process dies, so a crash leaves nothing to delete by hand. This is the guard
    that stops two instances fighting over the rig frequency.
  - update: simpler here. Unix renames over a running binary, so the deferred
    swap the Windows path needs a detached helper for is unreachable.
  - tasklist/taskkill become /proc and SIGTERM; the boot log moves out of /tmp,
    which is wiped exactly when the evidence is wanted.
  - serial ports sorted naturally: /dev/ttyUSB10 was landing between USB1 and
    USB2, the same trap COM10 fell into.

release.ps1 now cross-builds and vets for linux before it builds the exe, and
refuses the release if that fails — a port rots one unguarded x/sys/windows call
at a time.

Nothing has been executed on Linux yet: Wails needs webkit2gtk and cgo there, so
the binary must be built on Linux. scripts/linux-setup.sh checks the machine and
does it; BUILDING-LINUX.md is the manual version.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

datadir_linux.go

@@ -0,0 +1,74 @@
+//go:build linux
+
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+// systemInstallDataDir keeps the portable "data beside the binary" layout where
+// it works, and falls back to the XDG data directory where it cannot.
+//
+// Both halves are needed on Linux, and only on Linux. A tarball or AppImage
+// unpacked into the home directory behaves exactly like the Windows build —
+// the folder travels with the program, which is the whole point of the design.
+// But the ordinary way software arrives here is a package that installs into
+// /usr/bin or /opt, where no user may write, and telling an operator to "move
+// the program somewhere writable" is telling them their distribution installed
+// it wrong. So when the folder beside the binary is read-only, OpsLog keeps its
+// data in ~/.local/share/OpsLog instead and says so in the log.
+//
+// Detection is by TRYING, not by matching path prefixes: /opt, /usr/local and a
+// NFS-mounted home are all writable on some stations and not on others, and the
+// only honest test is whether the write succeeds.
+func systemInstallDataDir(besideExe string) (string, bool) {
+	xdgOnce.Do(func() { xdgDir, xdgUsed = resolveDataDir(besideExe) })
+	return xdgDir, xdgUsed
+}
+
+var (
+	xdgOnce sync.Once
+	xdgDir  string
+	xdgUsed bool
+)
+
+func resolveDataDir(besideExe string) (string, bool) {
+	if writable(besideExe) {
+		return "", false
+	}
+	base := os.Getenv("XDG_DATA_HOME")
+	if base == "" {
+		home, err := os.UserHomeDir()
+		if err != nil || home == "" {
+			return "", false // nowhere better to go; let the caller report the failure
+		}
+		base = filepath.Join(home, ".local", "share")
+	}
+	alt := filepath.Join(base, "OpsLog", "data")
+	if !writable(alt) {
+		return "", false
+	}
+	bootLog("data dir: %s is not writable — keeping the data in %s instead", besideExe, alt)
+	return alt, true
+}
+
+// writable reports whether dir can be created and written to. The probe file is
+// removed again; a leftover in the data folder would be one more thing to
+// explain.
+func writable(dir string) bool {
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return false
+	}
+	probe := filepath.Join(dir, ".writetest")
+	if err := os.WriteFile(probe, []byte("ok"), 0o644); err != nil {
+		return false
+	}
+	_ = os.Remove(probe)
+	return true
+}
+
+// dataDirAdvice is what the operator is told when neither location works — a
+// full disk, or a home directory that is not writable either.
+const dataDirAdvice = "\n\nOpsLog keeps its data next to the program, or in ~/.local/share/OpsLog when that folder belongs to the system. Neither could be written to: check the disk is not full and that your home directory is writable."