lang: fn aliases dispatch like their target (fix 0121) — scan-time registration through the shared alias-chain walk

Renamed fn aliases failed for EVERY kind (the filed pack-only scope was
a same-name confound: same-name re-exports already resolved through the
name-keyed fn_ast_map). scanDecls now follows ident-/ns.X-RHS const
alias chains (aliasedFnDecl; 0120's hop walk extracted as
followAliasChain) and registers the alias name in fn_ast_map
(absent-only), so every dispatch path — early pack/comptime/generic,
plain lazy-lower, plan-side typing — sees the target decl unchanged.
my_print :: s.print; / my_format :: s.format; now work (the std.sx
re-export shape). Regression: examples/0546 (+rich). Gates: zig build
test 0, suite 588/588.

src/ir/lower.zig

@@ -1688,6 +1688,7 @@ pub const Lowering = struct {
     pub const buildGenericStructTemplate = lower_nominal.buildGenericStructTemplate;
     pub const qualifiedStructTemplate = lower_nominal.qualifiedStructTemplate;
     pub const aliasedStructTemplate = lower_nominal.aliasedStructTemplate;
+    pub const aliasedFnDecl = lower_nominal.aliasedFnDecl;
     pub const qualifiedMemberMissing = lower_nominal.qualifiedMemberMissing;
     pub const bareVisibleStructDecl = lower_nominal.bareVisibleStructDecl;
     pub const bareVisibleStructTemplate = lower_nominal.bareVisibleStructTemplate;