The fn-body `#foreign [LIB] ["csym"]` marker now builds the SAME shape postfix
`extern` produces — extern_export = .extern_ + extern_lib/extern_name + an
empty-block body — instead of a `foreign_expr` body. With all four prereqs
landed (visibility, variadic, plain-free classification, lib-ref validation),
every downstream reader coalesces is_foreign with extern_export, so the IR and
runtime behavior are byte-identical (full corpus + the A->B gate stay green).
The surface keyword is no longer on the AST, so a `#foreign`-spelled decl now
yields `extern`-worded diagnostics — the single accepted churn (Decision 7):
example 1620's lib-ref error flips '#foreign library' -> 'extern library'.
Parser-surface diagnostics (conflict/expected-token) fire on the literal keyword
and are unaffected. c_import auto-synthesis still emits foreign_expr bodies (not
this step), so both shapes still coexist. Parser unit test updated to assert the
extern shape.
647 corpus / 444 unit, 0 failed. The const-with-type (dead) + runtime-class
(already coalesced) paths need no flip — Phase 5.0 parser routing is complete.
validateForeignRefs walks the merged tree (libraries + named c units,
nested namespaces included) and diagnoses any #foreign whose ref names
neither — a typo'd ref previously compiled and resolved silently
through whatever image carried the symbol. Decls synthesized from
#include headers carry no ref and are exempt. Flips the C0.2b pin;
zero collateral across the 608 other examples.
