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