cgen: resolve chained-DOT fn-ptr field callees

fnptrcalleetfn's N_DOT arm required an N_IDENT base, so a.b.cb(...)
fell to the name registry with an empty module hint and emitted
CALL cb(SB) (undefined symbol; cstage calls the stamped ptr indirect).
dotbasestructinfo resolves the base chain through the struct registry
— each link a struct- or *struct-typed field — and the single-dot
path routes through the same resolver unchanged. Closes the last open
shape of the #59.8 name-keyed callee family. Fixture fnptrfield_chain;
corpus pin 1487/2974.

docs/test-system-v2.md

@@ -32,9 +32,9 @@ categories out of the ordinary developer target.
 | Fixed point and self-host | `test-bootstrap` |
 | Host linker/platform behavior | `test-platform` |
 
-The live declarative compiler corpus has 1,486 fixtures and 2,972 C/WW cells:
+The live declarative compiler corpus has 1,487 fixtures and 2,974 C/WW cells:
 338 expected rejections (314 shared and 24 stage-specific), 17 compile-only
-successes, 191 exit-zero programs, and 940 explicit-exit programs.
+successes, 191 exit-zero programs, and 941 explicit-exit programs.
 
 147 native C carriers remain. They are partitioned exactly once as five
 in-process units, 24 byte/artifact gates, six bootstrap gates, one platform