fix(bif): set IFACE_VERSION to 2 for the v0.12.0 release (was 1 in public v0.11.x)

The .bif format was bumped 1->2->3 internally across this development cycle, but
the last PUBLIC commit (d56bdbf, release v0.11.x) shipped IFACE_VERSION = 1 with
COMPILER_ID +bif1.  None of the intermediate v2/v3 layouts ever reached a public
reader, so the cumulative format change since v1 is a SINGLE public step: collapse
it to v2.

- IFACE_VERSION 3 -> 2; comment now lists all the META/record additions made this
  cycle under v2 (ImplUsedUnits + HasInitialization, block local var decls,
  Exit value, parameter defaults, free-routine ResolvedQbeName, generic-class
  template properties, TRoutineSig vtable facts).
- COMPILER_ID stays blaise-0.12.0-SNAPSHOT+bif2 (now matches IFACE_VERSION).
- Magic-string test updated to expect 'BLAISE-IFACE 2'.

v1 .bif written by v0.11.x are rejected and recompiled from source, as intended.

compiler/src/main/pascal/uUnitInterfaceIO.pas

@@ -53,11 +53,21 @@ uses
 
 const
   IFACE_MAGIC   = 'BLAISE-IFACE';
-  IFACE_VERSION = 3;  { v2: META block carries ImplUsedUnits + HasInitialization
-                        v3: EncodeBlock carries local var declarations so generic
-                            template method bodies round-trip with their locals
-                            (e.g. 'var Ptr: ^T'); without this an instantiation
-                            from a cached .bif fails with 'Undeclared variable'. }
+  IFACE_VERSION = 2;  { v1: shipped through release v0.11.x (last public commit
+                            d56bdbf).
+                        v2 (release v0.12.0): a batch of META/record additions
+                          made this cycle, all gated by this single bump since
+                          v1 never reached a reader that lacked them:
+                            - META block carries ImplUsedUnits + HasInitialization
+                            - EncodeBlock carries local var declarations so generic
+                              template method bodies round-trip with their locals
+                              (e.g. 'var Ptr: ^T')
+                            - Exit(value) carries its return value
+                            - parameter default values are serialised
+                            - free-routine ResolvedQbeName, generic-class template
+                              properties, and TRoutineSig vtable facts
+                              (VTableSlot/IsVirtual/IsOverride) round-trip.
+                          Old (v1) .bif are rejected and recompiled from source. }
 
 type
   EIfaceFormatError = class(Exception);

compiler/src/test/pascal/cp.test.unitinterface.pas

@@ -2683,7 +2683,7 @@ begin
     Buf := WriteUnitInterface(Iface);
     { Blaise Pos is 0-based; match-at-start returns 0. }
     AssertTrue('starts with magic',
-      Pos('BLAISE-IFACE 3', Buf) = 0);
+      Pos('BLAISE-IFACE 2', Buf) = 0);
   finally
     Iface.Free();
   end;