docs: retire dead SBCL host surface; fix stale paths; add tests/READMEVerified

host-sbcl-legacy/ was deleted long ago, but funcl.asd still pointed at it
(so the ASDF system wouldn't even load), and CLAUDE.md + Makefile:10 still
called it 'retained'.  The elisp path reached self-host, so the SBCL host +
its FiveAM suite are fully retired:

- rm funcl.asd + the 8 vestigial FiveAM test files (tests/{package,smoke,
  asm-tests,compiler-tests,reader-tests,pe-tests,ast-ir-tests,bootstrap-
  tests}.lisp) — none referenced by any live target; funcl is Makefile-built.
- CLAUDE.md + Makefile: correct the host-sbcl-legacy 'retained' prose to
  'deleted'.
- Fix stale self-name headers: src/stdlib.lisp said src/prelude.lisp;
  src/cold/compiler.lisp said src/compiler.lisp (comment-only, byte-identical).
- Rewrite src/README.md for the real src/cold + src/warm + stdlib/mini-clos
  layout (was the old flat src/stage1.lisp/elf-tiny.lisp list); point at the
  Makefile as the authoritative source order; note self-host is ACHIEVED.
- Add tests/README.md: coverage map (targets, durations, gate vs iteration).

No source semantics changed; self-host fixed point holds (build byte-identical,
funcl 6231177 bytes unchanged).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>