Require ClassicAPI v1.15.0

An out-of-date ClassicAPI did not degrade the addon, it decapitated it.
Utility.lua calls ev:RegisterUnitEvent at file scope (v1.15.0), so on an
older build that call raises and Lua abandons the rest of the chunk --
every definition below line 4118 of a 10040-line file, some 6000 lines,
silently never happens. What the user sees is the first one a common path
reaches: PLAYER_TARGET_CHANGED calling a nil ClearResistState, once per
target swap, forever.

The requirement check still said v1.12.1 (the positional
C_UnitAuras.UnitAura), so the affected client sailed through it and the
warning it did print promised only that "dispel-type conditionals will be
unavailable" -- describing a graceful degradation that was not happening.
The gate is now v1.15.0 and says the addon cannot finish loading, with the
installed version in the message the way the Nampower branch does it.
ClassicAPI.lua gains GetVersion() to decode CLASSIC_API_VERSION, keeping
the X*10000 + Y*100 + Z encoding inside that module, and its header notes
that the minimum has to rise with any ClassicAPI call adopted at file
scope -- that is the property that turns a version mismatch into a silent
half-load rather than a missing feature.

C_Macro.SetMacroDisplay also ships in v1.15.0, which retires the last
pre-ClassicAPI display path:

  useClassicAPIDisplay folds into ClassicAPIMacroDisplay. The two were
  always equal -- set together at load, cleared together in
  ReleaseDisplays -- and one of them is a handshake ClassicAPI reads, so
  a second name for the same state could only ever drift.

  TestForAllActiveActions loses the per-slot ACTIONBAR_SLOT_CHANGED
  fan-out. Publishing repaints every slot holding the macro through the
  client's own notifier; the fan-out was what ran when SetMacroDisplay
  was unavailable, and its one remaining reachable state was after
  DisableAddon, where ReleaseDisplays has already handed all 36 macros
  back and ClassicAPI repaints them itself.

The load-time feature detect stays, with a new reason: below the minimum
the addon is already broken, so a nil SetMacroDisplay now means ClassicAPI
is absent outright -- the case the requirement check warns about and then
keeps running.

ClassicAPI.lua

@@ -3,11 +3,16 @@
 
     ClassicAPI is a client mod (sibling to Nampower/SuperWoW) that backports the
     modern C_* API into the 1.12.1 Lua environment. It is a HARD REQUIREMENT of
-    this addon (ClassicAPI v1.12.1+, which added the positional
-    C_UnitAuras.UnitAura), so the wrappers below call the API directly — no
-    fallbacks. The load-time requirement check (Core.lua) uses IsAvailable() to
-    warn when the DLL is missing and HasMinimumVersion() when it's too old; users
-    who don't want ClassicAPI should run the upstream addon.
+    this addon (ClassicAPI v1.15.0+, which added frame:RegisterUnitEvent), so the
+    wrappers below call the API directly — no fallbacks. The load-time requirement
+    check (Core.lua) uses IsAvailable() to warn when the DLL is missing and
+    HasMinimumVersion() when it's too old; users who don't want ClassicAPI should
+    run the upstream addon.
+
+    The minimum is not advisory: Utility.lua calls frame:RegisterUnitEvent at file
+    scope, so an older ClassicAPI aborts that chunk and leaves most of the addon
+    undefined. Raise the Core.lua minimum in step with any new API adopted at file
+    scope.
 
     Detection: the global CLASSIC_API_VERSION is defined once the client has
     booted, encoded as X*10000 + Y*100 + Z for a vX.Y.Z tag (untagged dev builds
@@ -37,6 +42,14 @@ function API.GetVersionNumber()
     return CLASSIC_API_VERSION or 0
 end
 
+-- Returns the loaded version as major, minor, patch (0, 0, 0 if absent).
+function API.GetVersion()
+    local v = CLASSIC_API_VERSION or 0
+    local major = math.floor(v / 10000)
+    local minor = math.floor(v / 100) - major * 100
+    return major, minor, v - math.floor(v / 100) * 100
+end
+
 -- True if the ClassicAPI client mod is loaded at all.
 function API.IsAvailable()
     return CLASSIC_API_VERSION ~= nil
@@ -56,7 +69,7 @@ end
 
 -- Scan one aura range of `unit` (filter = "HELPFUL" or "HARMFUL") for an aura
 -- matching the dispel type. Uses the positional C_UnitAuras.UnitAura (added in
--- ClassicAPI v1.12.1, this addon's minimum) -- no table allocated per slot, with
+-- ClassicAPI v1.12.1, below this addon's minimum) -- no table allocated per slot, with
 -- dispelName as the 4th return. The filtered index self-terminates at the end of
 -- the range (nil name); 48 is a backstop over vanilla's 32 helpful / 16 harmful slots.
 local function scanDispel(unit, filter, dispelType, wantAny)