fix(hotkey): split platform files by platform+cgo build matrix

The `!darwin` guard on the modifier helpers was too broad: golang.design/x/
hotkey only exposes its Mod*/Key* constants from the darwin&&cgo, linux&&cgo
and windows backends, while the `!windows && !cgo` complement (e.g. Linux
cross-builds with CGO_ENABLED=0) falls back to a stub with no constants. That
made hk.ModCtrl/ModShift/KeyA... undefined and produced editor/vet warnings.

Realign the build tags with the upstream matrix and confine every hk.* constant
to a tagged file so the shared hotkey.go compiles everywhere. Also fix the
Linux mapping (option -> hk.Mod1; hk.ModAlt does not exist on X11).

internal/infrastructure/hotkey/hotkey.go

@@ -104,7 +104,7 @@ func parseSpec(spec string) ([]hk.Modifier, hk.Key, error) {
 		case "option", "alt":
 			mods = append(mods, modOption())
 		case "shift":
-			mods = append(mods, hk.ModShift)
+			mods = append(mods, modShift())
 		default:
 			k, ok := lookupKey(p)
 			if !ok {

internal/infrastructure/hotkey/hotkey_darwin.go

@@ -1,10 +1,15 @@
-//go:build darwin
+//go:build darwin && cgo
 
 package hotkey
 
 import hk "golang.design/x/hotkey"
 
 // macOS uses ⌘ as the primary modifier; Option corresponds to Alt.
+//
+// This file is gated on cgo because golang.design/x/hotkey only exposes the
+// Mod* constants from its cgo-backed darwin implementation; with CGO_ENABLED=0
+// the package falls back to a stub that defines no constants.
 func modCmd() hk.Modifier    { return hk.ModCmd }
 func modCtrl() hk.Modifier   { return hk.ModCtrl }
 func modOption() hk.Modifier { return hk.ModOption }
+func modShift() hk.Modifier  { return hk.ModShift }

internal/infrastructure/hotkey/hotkey_linux.go

@@ -0,0 +1,18 @@
+//go:build linux && cgo
+
+package hotkey
+
+import hk "golang.design/x/hotkey"
+
+// On Linux we map "cmd" to Ctrl so the same config string ("cmd+shift+a")
+// stays meaningful across platforms.
+//
+// The X11 backend exposes modifiers as Mod1..Mod5 rather than named Alt/Option
+// constants; Mod1 is the conventional Alt mask under X11, so "option"/"alt"
+// resolve to hk.Mod1. This file is gated on cgo because the constants only
+// exist in the cgo-backed linux implementation (CGO_ENABLED=0 falls back to a
+// stub with no constants).
+func modCmd() hk.Modifier    { return hk.ModCtrl }
+func modCtrl() hk.Modifier   { return hk.ModCtrl }
+func modOption() hk.Modifier { return hk.Mod1 }
+func modShift() hk.Modifier  { return hk.ModShift }

internal/infrastructure/hotkey/hotkey_other.go

@@ -1,11 +0,0 @@
-//go:build !darwin
-
-package hotkey
-
-import hk "golang.design/x/hotkey"
-
-// On Windows / Linux we map "cmd" to Ctrl so the same config string works
-// across platforms; "option" is treated as Alt.
-func modCmd() hk.Modifier    { return hk.ModCtrl }
-func modCtrl() hk.Modifier   { return hk.ModCtrl }
-func modOption() hk.Modifier { return hk.ModAlt }

internal/infrastructure/hotkey/hotkey_unsupported.go

@@ -0,0 +1,25 @@
+//go:build !windows && !cgo
+
+package hotkey
+
+import hk "golang.design/x/hotkey"
+
+// This file covers the configuration where golang.design/x/hotkey compiles its
+// cgo-less stub (any non-Windows platform built with CGO_ENABLED=0). In that
+// stub the package defines the Modifier/Key types but no Mod*/Key* constants,
+// and Register panics at runtime. We still provide the package-internal
+// helpers so that snapgo keeps cross-compiling (e.g. `go vet`, editor analysis,
+// or CI cross-builds) without pulling in a C toolchain.
+//
+// The returned values are inert zero modifiers and lookupKey always reports
+// "unsupported": callers on this build cannot register a real global hotkey,
+// so surfacing an "unsupported token" parse error is preferable to reaching
+// the upstream panic.
+func modCmd() hk.Modifier    { return 0 }
+func modCtrl() hk.Modifier   { return 0 }
+func modOption() hk.Modifier { return 0 }
+func modShift() hk.Modifier  { return 0 }
+
+// lookupKey has no key constants to map against in the cgo-less stub, so it
+// always reports the token as unsupported.
+func lookupKey(string) (hk.Key, bool) { return 0, false }

internal/infrastructure/hotkey/hotkey_windows.go

@@ -0,0 +1,13 @@
+//go:build windows
+
+package hotkey
+
+import hk "golang.design/x/hotkey"
+
+// On Windows we map "cmd" to Ctrl so the same config string ("cmd+shift+a")
+// stays meaningful across platforms; "option" is treated as Alt. The Windows
+// backend does not require cgo, hence the single-tag guard.
+func modCmd() hk.Modifier    { return hk.ModCtrl }
+func modCtrl() hk.Modifier   { return hk.ModCtrl }
+func modOption() hk.Modifier { return hk.ModAlt }
+func modShift() hk.Modifier  { return hk.ModShift }

internal/infrastructure/hotkey/keys.go

@@ -1,9 +1,15 @@
+//go:build windows || cgo
+
 package hotkey
 
 import hk "golang.design/x/hotkey"
 
 // lookupKey maps a token from the user-supplied hotkey spec to a hk.Key.
 //
+// The Key* constants only exist in the platform backends that ship them
+// (windows, or the cgo-backed darwin/linux implementations); the cgo-less stub
+// in hotkey_unsupported.go provides a fallback lookupKey for the complement.
+//
 // The map is intentionally exhaustive over the alphanumeric set so we never
 // rely on an assumption that hk.KeyA..hk.KeyZ are contiguous values — the
 // upstream package gives no such guarantee.