refactor: rename src/prelude.lisp -> src/stdlib.lisp (owner directive)Verified

The file outgrew its name: ~7000 lines holding the entire standard
library (CL stdlib, FORMAT, conditions/restarts, packages, eval,
typep/deftype, file streams). "Prelude" implied a small implicitly-
loaded warm-up; this is the stdlib.

File-level rename only: git mv + every functional path reference —
cold-bootstrap's CWD open + error strings, main.lisp fallback comment,
stage1/dwarf contract comments, Makefile dependency, tests/bootstrap.el
Phase-4 symlink fixture, CLAUDE.md. Internal ag-*prelude* identifiers
and historical ADR texts intentionally untouched (that sweep is a
separate, fixed-point-sensitive change).

Gates: elisp-bytecode; build=0 (stage1+stage2 self-compile against the
new path); probe battery green (defstruct/labels/io round-trip/
copy-file/check-type); --hosted 42/105; test-fast-sharded ALL 8 SHARDS
PASSED (including the stdlib.lisp symlink fixture).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>