Implement VIEW/WINDOW/PALETTE, PMAP, fix MBF float format, update to v0.13.0

Graphics viewport and coordinate mapping:
- VIEW [[SCREEN] (x1,y1)-(x2,y2) [,[fill][,border]]] with clipping
- WINDOW [[SCREEN] (x1,y1)-(x2,y2)] with Cartesian/screen modes
- PALETTE [attribute, color] with CGA 16-color remapping
- PMAP(coord, func) for logical/physical coordinate conversion
- All graphics statements (PSET, LINE, CIRCLE, PAINT, GET/PUT) respect
  viewport clipping and WINDOW coordinate mapping

MBF (Microsoft Binary Format) float support:
- CVS/CVD now interpret bytes as MBF format (compatible with real GW-BASIC)
- MKS$/MKD$ now produce MBF-encoded bytes
- Fixed shift errors in MBF↔IEEE conversion routines (single: 1→0, double: 4→3)
- Random-access file I/O now byte-compatible with original GWBASIC.EXE

66 tests (2 new), 61 compat matches (up from 58).

tests/expected/mbf_format.expected

@@ -0,0 +1,15 @@
+ 0
+ 1
+-1
+ 3.14
+ 100.5
+ 0
+ 1
+-1
+ 3.14
+ 0
+ 32767
+-32768
+-1
+ 0  0  0  129
+DONE

tests/expected/view_window.expected

@@ -0,0 +1,7 @@
+PMAP(0,0)= 0
+PMAP(0,1)= 199
+PMAP(100,0)= 639
+PMAP(100,1)= 0
+PMAP(0,2)= 0
+PMAP(199,3)= 0
+VIEW/WINDOW/PALETTE OK

tests/programs/mbf_format.bas

@@ -0,0 +1,22 @@
+10 REM MBF format conversion test
+20 REM Test MKS$/CVS roundtrip
+30 PRINT CVS(MKS$(0))
+40 PRINT CVS(MKS$(1))
+50 PRINT CVS(MKS$(-1))
+60 PRINT CVS(MKS$(3.14))
+70 PRINT CVS(MKS$(100.5))
+80 REM Test MKD$/CVD roundtrip
+90 PRINT CVD(MKD$(0#))
+100 PRINT CVD(MKD$(1#))
+110 PRINT CVD(MKD$(-1#))
+120 PRINT CVD(MKD$(3.14#))
+130 REM Test MKI$/CVI roundtrip
+140 PRINT CVI(MKI$(0))
+150 PRINT CVI(MKI$(32767))
+160 PRINT CVI(MKI$(-32768))
+170 PRINT CVI(MKI$(-1))
+180 REM Test MBF byte encoding
+190 REM MKS$(1) in MBF should be: 00 00 00 81 (exponent=129, mantissa=0.5)
+200 A$ = MKS$(1)
+210 PRINT ASC(MID$(A$,1,1)); ASC(MID$(A$,2,1)); ASC(MID$(A$,3,1)); ASC(MID$(A$,4,1))
+220 PRINT "DONE"

tests/programs/view_window.bas

@@ -0,0 +1,21 @@
+10 REM VIEW / WINDOW / PALETTE test
+20 SCREEN 2
+30 REM Test WINDOW coordinate mapping
+40 WINDOW (0,0)-(100,100)
+50 REM PMAP: logical to physical
+60 PRINT "PMAP(0,0)="; PMAP(0,0)
+70 PRINT "PMAP(0,1)="; PMAP(0,1)
+80 PRINT "PMAP(100,0)="; PMAP(100,0)
+90 PRINT "PMAP(100,1)="; PMAP(100,1)
+100 REM PMAP: physical to logical
+110 PRINT "PMAP(0,2)="; PMAP(0,2)
+120 PRINT "PMAP(199,3)="; PMAP(199,3)
+130 WINDOW
+140 REM Test VIEW
+150 VIEW (10,10)-(100,50)
+160 REM After VIEW, drawing clips to viewport
+170 VIEW
+180 REM Test PALETTE (just reset, no visual check)
+190 PALETTE
+200 SCREEN 0
+210 PRINT "VIEW/WINDOW/PALETTE OK"