feat(ffi-linkage): extern/export parser+AST plumbing, unconsumed (Phase 0.1)

Add ast.ExternExportModifier { none, extern_, export_ } beside
CallingConvention; FnDecl.extern_export and VarDecl.is_extern/extern_name
fields (all defaulting to absent); and Parser.parseOptionalExternExport()
mirroring parseOptionalCallConv.

None of this is consumed by a decl path yet — no user-facing behavior
change, corpus diff empty. Two inline parser unit tests pin the helper's
keyword mapping and the field defaults. Phase 1.0 wires the helper into
the fn-decl path. lock commit.

src/ast.zig

@@ -123,6 +123,14 @@ pub const Root = struct {
 
 pub const CallingConvention = enum { default, c };
 
+/// Linkage modifier written in the postfix slot after `callconv(...)`:
+/// `name :: (sig) -> Ret [callconv(.x)] [extern | export] [;|{…}];`
+/// `extern` = import (external linkage, C ABI, no sx ctx — `#foreign`'s role);
+/// `export` = define + expose (body + external linkage + C ABI + no ctx).
+/// Both imply `callconv(.c)`. Variants carry a trailing `_` to dodge the Zig
+/// keywords. `.none` = no linkage modifier (the ordinary sx-internal decl).
+pub const ExternExportModifier = enum { none, extern_, export_ };
+
 pub const FnDecl = struct {
     name: []const u8,
     params: []const Param,
@@ -131,6 +139,10 @@ pub const FnDecl = struct {
     type_params: []const StructTypeParam = &.{},
     is_arrow: bool = false,
     call_conv: CallingConvention = .default,
+    /// Postfix linkage modifier (`extern`/`export`) written after the
+    /// `callconv(...)` slot. `.none` for an ordinary sx-internal function.
+    /// Parsed in Phase 0.1; not consumed by the fn-decl path until Phase 1.
+    extern_export: ExternExportModifier = .none,
     /// Span of the function's name token, for the reserved-type-name decl
     /// diagnostic. Synthesized decls (e.g. `#import c` foreign
     /// functions, lowering-time objc/protocol method synthesis) leave it zero.
@@ -343,6 +355,13 @@ pub const VarDecl = struct {
     is_foreign: bool = false,
     foreign_lib: ?[]const u8 = null,
     foreign_name: ?[]const u8 = null,
+    /// `extern`-global form `g : T extern ["csym"];` — a reference to a global
+    /// defined elsewhere (external linkage, resolved at link time). The new
+    /// extern-named surface; distinct from the legacy `#foreign` path above.
+    /// `extern_name` is the optional symbol-name override. Parsed in Phase 0.1;
+    /// not consumed by the var-decl path until Phase 1.2.
+    is_extern: bool = false,
+    extern_name: ?[]const u8 = null,
     /// True when the binding name was written as a backtick raw identifier
     /// (`` `i2 := … ``). A raw name is exempt from the reserved-type-name
     /// binding check.