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.

examples/expected/0546-packs-fn-alias.exit

@@ -0,0 +1 @@
+0

examples/expected/0546-packs-fn-alias.stderr

@@ -0,0 +1 @@
+

examples/expected/0546-packs-fn-alias.stdout

@@ -0,0 +1,6 @@
+pack: 7
+plain: 7
+generic: 10
+ns-pack: 42
+std-print: 1 two
+std-format 42