feat: primitive append + rename host/ to elisp/Verified

Two related cleanups landed together.

emit-ir-append as a primitive: ,@ no longer relies on an auto-
prepended recursive defun. The root cause of the prior failure was
asm.lisp's rel8 helpers using `(logand delta 255)`, which isn't in
the bootstrap arithmetic subset — so host-direct compilation
worked but stage1's self-built copy mis-emitted any rel8 jump.
Replaced with u8-wrap built from rem + add. With that in place,
the primitive (and the workaround removal in stage1's main and the
elisp host's prelude-prepend) compose cleanly. A new stage1 case
exercises rel8 via apply.

Directory rename: host/ is now elisp/. The legacy SBCL host lives
at host-sbcl-legacy/, so "host" stopped being a useful
disambiguator; per the user's directional preference the elisp
host is permanent, so the dir name reflects what it is. All path
references chased through code, comments, Makefile, CLAUDE.md, and
src/README.md. CLAUDE.md's stale "**Host**: SBCL 2.4.0 today"
block rewritten to match reality.

120/120 ert green; make elisp-smoke writes a runnable exit-42 ELF.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>