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_source/defining_words.c

@@ -478,6 +478,7 @@ static void defining_word_variable(VM *vm) {
         return;
     }
 
+    vm_align(vm);
     vaddr_t addr = (vaddr_t) vm->here;
     void *p = vm_allot(vm, sizeof(cell_t));
     if (!p) {