starkernel: item 4.3.3 -- Cartesian coordinate machinery, found and fixed a VARIABLE alignment bug

Adds Module 28 (framebuffer_words.c/.h): PLOT ( x y color -- ), FB-WIDTH,
FB-HEIGHT -- raw hardware-boundary C primitives, kernel-only, no-op on
hosted builds, same pattern as every other module.

Adds capsules/fabric.4th (blocks 4900-4902, mkcapsule --lint clean):
COS45/Z->DELTA/PROJECT/CART-Y/CART-PLOT -- the 45-degree cavalier
orthographic projection and Y-flip, in FORTH per the compose-in-FORTH-first
rule (this is policy, not hardware access).

Found and fixed a second real bug while live-testing CART-PLOT over the
serial socket: defining_word_variable() (defining_words.c) captured
vm->here as a VARIABLE's address with no alignment call first, while
vm_load_cell/vm_store_cell require 8-byte-aligned addresses. This capsule's
VARIABLE ZD landed misaligned (945) purely by chance of what preceded it;
other capsules' variables happened to land aligned by luck, not guarantee.
Real deviation from FORTH-83/ANS, which specifies VARIABLE reserves an
aligned cell. Fixed with vm_align(vm) before capturing addr -- ALIGN
already existed as a word but VARIABLE wasn't calling it.

Verified end-to-end on amd64 via manual serial injection + QEMU screendump:
plotted 4 marker points (origin, +100 X, +100 Y, +50 Z) and confirmed all
landed at hand-calculated raster coordinates, including the diagonal
up-right shift for the Z-axis point -- the projection math is correct, not
just non-crashing. fb/fabric-test-cart-plot.png. 4.3.1's corner diagnostic
still renders correctly in the same shot, confirming no regression.

All three architectures (amd64/aarch64/riscv64) boot clean to ok> with the
DoE completing; dict_hash identical across all three
(0xc7f9adf885e306d2), confirming parity is unaffected.

FABRIC.md item 4.3.3 marked done with full acceptance evidence.

src/word_registry.c

@@ -73,6 +73,7 @@
 #include "word_source/include/q48_words.h"
 #include "word_source/include/inference_words.h"
 #include "word_source/include/defer_words.h"
+#include "word_source/include/framebuffer_words.h"
 
 /**
  * @brief Registers a single FORTH word in the virtual machine
@@ -128,6 +129,7 @@ void register_forth79_words(VM *vm) {
     register_q48_words(vm); /* Module 25: Q48.16 Fixed-Point Math */
     register_inference_words(vm); /* Module 26: SSM Inference + Jacquard */
     register_defer_words(vm);     /* Module 27: DEFER / IS late binding */
+    register_framebuffer_words(vm); /* Module 28: Console fabric -- raw framebuffer primitives */
 
     log_message(LOG_INFO, "FORTH-79 Standard word set registration complete");
 }