feat(selfhost): AST dump span-tier scaffolding — oracle heads to push_node, tier 0 unchanged

Convert every spanned AST node HEAD line in the oracle serializer
(ast_oracle_dump / dump_*) from push_line to push_node, threading the
node's Span. At the default AST tier 0 sp() returns "" so the output is
byte-identical; ast_parity_corpus stays 619/619 byte-exact (verified).

Spanned per the confirmed .span sources: items (module/use/type/sum+variant/
type-alias/const/fn), generic/field/param/type-ref, match head + arm, every
Stmt variant head (structs + malformed payload spans + break/continue/unknown),
and every Expr head via e.span() (binary op token, try ? token, receiver-call
receiver token, etc. as the parser sets them). Synthetic label lines
(value/body/block/cond/then/else/pattern/guard/callee-*/arg/object-field/…)
stay label-only.

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

Author: olwang