feat(plugins): Add a plugin hook framework for observer overlays

[doopey655]

Description

Adds a host-side plugin loader and a C ABI so an observer overlay can live outside the engine as a
DLL. The engine loads every *.goplugin.dll under plugins\ at startup; each plugin opts into the
hook categories it wants and receives events through an ABI that never exposes engine types.

With no plugin present nothing changes. Every call site is a guarded call whose condition is an
inlined !empty() on a static vector.

What it adds

• PluginABI.h — the host/plugin contract. Pure C (POD structs, function pointers,
  primitives), no STL and no engine types, so a plugin compiles against this one header. Versioned
  (GO_PLUGIN_ABI_VERSION 5) with a structSize guard; a mismatch fails the load rather than risk
  misreading the table. Exports GO_Plugin_GetInfo / _Initialize / _Shutdown, plus an optional
  _Tick.
• GOPluginManager — loads one folder per plugin, validates the ABI version, dispatches events.
  A plugin that fails to load is logged and skipped, never fatal.
• Gameplay hooks — unit and upgrade queued / cancelled / completed, building destroyed, special
  power triggered, object damaged and healed. Payloads carry template names as strings and object
  ids as uint32, never engine pointers.
• Render hooks — a per-frame overlay draw, plus raw key and mouse input delivered as a side
  channel that never consumes or gates the engine's own input handling.
• Host API — 2D text / rect / line / icon primitives, screen size, world-to-screen, object
  screen bounds, logic frame and FPS, the player roster with colours, general-power recharge state,
  viewport teleport, the per-user data path, and logging.

Observer-only gate

Callbacks carry information about other players, so the whole framework is gated on
IsLocalPlayerObserver() — local player is an observer or dead, which is also true during replay
playback. While that gate is closed no callback is delivered and GO_Plugin_Tick is not called, so
a match participant cannot gain an information advantage from a plugin. Same condition
InGameUI::drawObserverStats already uses.

Engine impact

17 files, ~2460 insertions, 0 deletions. No existing engine function is restructured — every
touch point is an inserted guarded call, so git diff main -U0 | grep '^-[^-]' is empty.

Three quarters of the diff is the two new self-contained files. The rest:

File	Added	What
PluginABI.h, PluginManager.h/.cpp	2076	new files
ProductionUpdate.cpp	82	queue / cancel / complete / destroy events
WindowXlat.cpp	70	mouse passthrough
InGameUI.cpp/.h	70	draw hook at the end of postWindowDraw(), plus two text primitives
ActiveBody.cpp	41	damage / heal events
View.h, W3DView.h/.cpp	40	worldToScreenTriReturnAllowFarClip(), below
CommandXlat.cpp	18	raw key-up passthrough
SpecialPowerModule.cpp	16	special-power event
W3DDisplay.cpp	16	registers the D3D device and HWND with the framework
GameEngine.cpp	10	load in init(), tick in update(), unload in the destructor
CMakeLists.txt	3	the new files

Two additions worth a reviewer's attention:

• View::worldToScreenTriReturnAllowFarClip() — a new virtual with a non-pure default, so
  existing View subclasses are unaffected. worldToScreenTriReturn rejects anything past the far
  clip plane, but CameraClass::Project has already written a valid perspective-divided position by
  then (only OUTSIDE_NEAR_CLIP zeroes its output), and an off-screen edge indicator needs it. It
  is deliberately standalone rather than sharing a body with the existing function, which is
  therefore untouched. A downcast from PluginManager was rejected: ViewDummy is installed as
  TheTacticalView in headless builds.
• InGameUI::drawPluginText2D[Scaled] — on InGameUI only because m_messageFont,
  m_messagePointSize and m_messageBold are protected with no getters. The rect and line
  primitives, which only forward to TheDisplay, live in the plugin framework instead.