feat(view): D4 Design View action family + reapply-on-open wiring

Register toggle/activate/tag/untag/show-both actions driving the D2
shell and D3 model; reapply saved active mode on project load via a
persist load-signal seam. Shared guidString helper; pure nextModeId
unit-tested.

src/actions.h

@@ -0,0 +1,42 @@
+#pragma once
+// actions — the Design View action family (Phase D4). Registers the bindable
+// actions that drive the mode workflow and wires them end-to-end: toggle/activate
+// a mode, tag/untag/show-both the current track selection. Each action mutates the
+// session's ViewModeModel (D1, via persist's ReaSamplerSession) and then reapplies
+// the active mode through the view shell (D2) so the change takes effect immediately.
+//
+// REAPER-facing shell: the .cpp includes reaper_plugin_functions.h WITHOUT
+// REAPERAPI_IMPLEMENT (main.cpp owns the API pointers — CLAUDE.md §contract). This
+// header is SDK-free; main.cpp calls register/handle/unregister and nothing else.
+//
+// Split out of main.cpp (rather than inlined there) to match CONTEXT.md's planned
+// `actions` module and keep main.cpp's entrypoint focused on API-pointer ownership
+// and lifecycle. The reapply-on-open glue stays in main.cpp (it owns the timer that
+// drives persist.poll()); this module only registers and services the actions.
+
+// Forward-declared at GLOBAL scope (matches reaper_plugin.h's typedef struct
+// reaper_plugin_info_t) so this header stays SDK-free; the .cpp includes the real
+// definition. Declared before the namespace so it is the global type, not a
+// namespace-local shadow.
+struct reaper_plugin_info_t;
+
+namespace reasampler {
+
+class ReaSamplerSession;
+
+// Registers the Design View action family against `rec` (command_id + gaccel +
+// hookcommand-routing is owned by the caller's single hookcommand). `session` is the
+// live session the actions mutate; it must outlive the registration. Idempotent is
+// NOT promised — call exactly once at load, mirror-unregister once at unload.
+void designViewRegisterActions(reaper_plugin_info_t* rec, ReaSamplerSession* session);
+
+// Services one fired command. Returns true iff `command` is one of this module's
+// action ids (and it was handled); false otherwise so the caller's hookcommand keeps
+// looking (per the contract: claim only our own ids). Safe to call for any command.
+bool designViewHandleCommand(int command);
+
+// Mirror-unregisters everything designViewRegisterActions registered, with the
+// '-'-prefixed strings (per the contract's unload rule). Call once on rec==nullptr.
+void designViewUnregisterActions(reaper_plugin_info_t* rec);
+
+}  // namespace reasampler