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/development.md

@@ -16,10 +16,11 @@
 | 0.10.0 | | Binary tokenized SAVE/LOAD, INKEY$ extended key sequences, golden-file regression tests, classic BASIC programs |
 | 0.11.0 | | DEF SEG / PEEK / POKE virtual memory, GET/PUT graphics sprites, PRINT USING fixes |
 | 0.12.0 | | BSAVE/BLOAD, TUI ANSI 16-color rendering, CGA graphics framebuffer PEEK/POKE, keyboard shift flags |
+| 0.13.0 | | VIEW/WINDOW/PALETTE graphics, PMAP function, MBF float format for CVS/CVD/MKS$/MKD$ |
 
 ## Tests
 
-64 test programs in `tests/programs/`, plus 4 classic interactive programs in
+66 test programs in `tests/programs/`, plus 4 classic interactive programs in
 `tests/classic/` (Hamurabi, Lunar Lander, Gunner, Diamond from David Ahl's
 *BASIC Computer Games*).
 
@@ -29,7 +30,7 @@ Run the full automated suite:
 bash tests/run_tests.sh
 ```
 
-Each test has a 5-second timeout. 60 tests have `.expected` golden files
+Each test has a 5-second timeout. 62 tests have `.expected` golden files
 for output regression detection. Three timing-dependent tests (datetime,
 on_timer, timer_stop) and one visual test (color_test) run without golden
 comparison.