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).

docs/roadmap.md

@@ -15,8 +15,6 @@
   bus so retro programs that talk to the speaker, joystick port, or parallel
   interface can do *something* useful instead of silently no-oping.  Think of it
   as a tiny museum exhibit for vintage BASIC hardware hacking.
-- **VIEW / WINDOW / PALETTE** — graphics viewport, coordinate mapping, and
-  palette remapping
 - **MBF format support for binary LOAD** — convert Microsoft Binary Format
   float constants when loading files saved by the original GWBASIC.EXE
 
@@ -36,8 +34,6 @@
 
 ## Known Limitations
 
-- Binary files use IEEE 754 floats, not MBF — files saved here are not
-  byte-compatible with original GWBASIC.EXE binary format
 - String garbage collection not implemented (uses `malloc`/`free` instead)
 - Maximum 256 variables, 64 arrays, 16 FOR nesting, 24 GOSUB nesting,
   16 WHILE nesting