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.

specs.md

@@ -1270,6 +1270,23 @@ chain resolves with the visibility of the file that declares THAT hop,
 not the use site's. Not yet supported: a qualified head whose namespace
 member is itself an alias (`ns.BoxAlias(..)`).
 
+### Function Aliases
+
+Functions alias the same way — bare or namespace-member RHS, renamed or
+same-name — and the alias dispatches exactly like the target. This covers
+every fn kind: plain, runtime-generic (`[]$T` / `$T: Type`), and
+comptime-pack (`..$args`, e.g. `print` / `format`):
+
+```sx
+s :: #import "modules/std.sx";
+my_print :: s.print;          // comptime-pack fn through a namespace
+helper2  :: r.helper;         // renamed plain fn
+my_print("x = {}\n", helper2());
+```
+
+(For making an alias *dot-callable*, see `name :: ufcs target;` in the
+UFCS section — that is a separate, explicit opt-in.)
+
 ### Generic Functions (Monomorphization)
 Functions can be parameterized over types using `$T` syntax. The `$` prefix introduces a type parameter; subsequent uses of the name reference it.
 ```sx