feat(materializer): native bulk byte-copy — byte-vector-append in cold sourceVerified

Phase 3a was blocked: the door-switch placement copies *agz-static-area*
into the code buffer via byte-vector-append, which existed only in
elisp/compat.el — unbound under a funcl-built stage, and the recursive
bv-copy-loop alternative overflows the stack on section-sized buffers.

Define byte-vector-append in src/cold/buffer.lisp as a tagbody/GO loop:
go compiles to a native jmp and byte-vector-push / byte-vector-byte are
inline primitives, so the copy is native-speed constant-stack. compat.el
gains a tagbody -> cl-tagbody alias so the elisp host runs the same
source; elisp/codegen.lisp restores the host's bulk unibyte-buffer
append after cold sources load (elf.lisp wraps MB-sized code buffers).

Verified flag-OFF: make build clean; REPL probes append 3 bytes and a
1 MiB buffer (constant stack) under build/funcl.

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