Add capsules/sdk.4th: SDK v1.9.0 vocabulary + FENCE over the cookbook capsules

Loads turtle.4th and doe.4th, defines SDK-VERSION/SDK-HELP into an SDK
vocabulary, then calls FENCE once everything is loaded -- protecting the
base wordset and both cookbook capsules from FORGET. Kernel-only (EXEC
doesn't exist hosted), REPL-invoked via S" sdk.4th" EXEC, not part of
init.4th's boot sequence.

Verified before writing the capsule, not assumed: VOCABULARY/DEFINITIONS
does not actually scope word visibility in this interpreter -- vm_find_word
is a flat dictionary scan that never consults CONTEXT/CURRENT. Documented
plainly in the HOWTO so this isn't mistaken for namespace isolation later.

Block range 5109-5115 -- discovered along the way that user-block space is
capped at [2048, 5120) by mkcapsule, tighter than expected.

Verified: mkcapsule --lint clean, hosted-build trace runs SDK-HELP with
zero attributable VM errors, zero build warnings and identical 1012/0/0
POST results with matching dict_hash on all three kernel architectures.

HOWTO: docs/working/architecture/SDK-HOWTO-20260819.md

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

capsules/sdk.4th

@@ -0,0 +1,60 @@
+Block 5109
+( sdk.4th -- SDK v1.9.0 scoping. Loads the cookbook       )
+( capsules (turtle.4th, doe.4th), a discoverable SDK      )
+( vocabulary entry point, and raises FENCE once           )
+( everything is loaded, so a developer at the REPL can    )
+( FORGET their own scratch work afterward without         )
+( disturbing any of this. Kernel-only (EXEC is kernel-    )
+( only); REPL-invoked -- S" sdk.4th" EXEC -- not part of  )
+( init.4th's boot sequence, matching turtle.4th's own     )
+( choice. NOTE: VOCABULARY/DEFINITIONS here is            )
+( organizational, not isolating -- this VM's word lookup  )
+( is a flat dictionary scan, so SDK words remain           )
+( globally callable like any other word, both before and  )
+( after this capsule loads. Verified directly on the       )
+( hosted build before writing this file.                   )
+VOCABULARY SDK
+
+Block 5110
+( Load the cookbook capsules this SDK re-exports. )
+S" turtle.4th" EXEC
+S" doe.4th" EXEC
+
+Block 5111
+( SDK's own words, defined into the SDK vocabulary for   )
+( introspection -- see the note above, they are just as  )
+( callable from FORTH context as anywhere else.          )
+SDK DEFINITIONS
+
+: SDK-VERSION ( -- )
+  ." SDK v1.9.0 (scoping)" CR ;
+
+Block 5112
+: SDK-HELP ( -- )
+  ." SDK vocabulary: turtle.4th + doe.4th + FENCE." CR
+  ." Turtle graphics (docs/working/architecture/" CR
+  ."   TURTLE-GRAPHICS-HOWTO-20260819.md):" CR
+  ."   HOME CS FORWARD BACK LEFT RIGHT PENUP PENDOWN" CR
+  ."   SETCOLOR SETXY SETHEADING POLYGON STAR TURTLE-DEMO" CR
+
+Block 5113
+  ." DoE library (docs/working/architecture/" CR
+  ."   DOE-LIBRARY-HOWTO-20260819.md):" CR
+  ."   DOE ( -- )  or  seed n-reps EXEC-DOE" CR
+  ." SDK-VERSION prints the release tag." CR ;
+
+Block 5114
+( Back to FORTH context -- CURRENT no longer targets  )
+( SDK, so anything typed at the REPL after this loads  )
+( defines normally.                                    )
+FORTH DEFINITIONS
+
+Block 5115
+( Raise FENCE now that the base wordset, turtle.4th,  )
+( doe.4th, and this file's own words are all loaded -- )
+( a developer can FORGET their own later scratch       )
+( definitions without being able to reach back          )
+( through any of this.                                  )
+FENCE
+SDK-VERSION
+." Loaded. Run SDK-HELP for the word list." CR