Per-subsystem hook control: install/remove input, focus, audio at runtime

Add a host->hook control channel (protocol v5 -> v6: HookControl in SharedBlock,
per-subsystem "disabled" flags, 0 = install so the zero-filled default is
unchanged). The worker now reconciles each subsystem every tick: install what's
requested-and-missing, remove what's no longer wanted -- so the audio hooks
re-attach the ring and republish format on a reinstall, and XInput/focus clear
their stale status flags on removal.

Injection panel: a checkbox per subsystem (input forwarding / focus spoof /
audio render-hook) toggles it at runtime, showing the requested vs actual
installed state from the registry, plus DLL heartbeat liveness. The hook-status
section now keys off whether a DLL was injected (host-side) rather than the
input-hook "attached" flag, so it stays visible with input unhooked.

Guards for dependent features: the synthetic-input control is disabled when
input forwarding is off, and the Audio panel explains that mirroring uses
loopback (echo) when the render-hook is off.

Verified against Phantom Brave via coop_audio_probe: starting with audio
requested off installs only input+focus (8 hooks, no capture); re-enabling at
runtime installs the audio hooks (13) and capture starts immediately. All four
tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

common/include/coop/protocol.hpp

@@ -11,7 +11,7 @@ namespace coop
 
 // Bump whenever the layout of SharedBlock or CoopPadState changes. The hook
 // refuses to attach to a host with a mismatched version.
-inline constexpr std::uint32_t kProtocolVersion = 5;
+inline constexpr std::uint32_t kProtocolVersion = 6;
 
 // 'COOP' little-endian, used to sanity-check the mapping before trusting it.
 inline constexpr std::uint32_t kProtocolMagic = 0x504F4F43u;
@@ -126,6 +126,14 @@ struct HookStatus
 	HookEntry hook_entries[kMaxHookEntries];
 };
 
+// Host -> hook control channel. The host requests which hook subsystems should be
+// installed; the hook reconciles each tick. 0 = install (the zero-filled default,
+// so a fresh mapping installs everything as before), 1 = remove.
+struct HookControl
+{
+	std::atomic<std::uint32_t> subsystem_disabled[HookSubsys_Count];
+};
+
 // Top-level shared block. The host is the sole writer of pad state; the hook is
 // the sole reader. A seqlock (even = stable, odd = write in progress) lets the
 // reader grab a torn-free snapshot without a kernel lock on the hot path.
@@ -140,6 +148,9 @@ struct SharedBlock
 	// Hook -> host diagnostics back-channel.
 	HookStatus status;
 
+	// Host -> hook control (which subsystems to install).
+	HookControl control;
+
 	// Phase 2 appends the shared-texture handle/dimensions control fields here;
 	// keep new members at the end so existing offsets never shift.
 };