fix(lower): resolve cross-module same-name functions by identity [0100]

Two modules each exporting a top-level function with the same short name
(std.cli.parse 3-param, std.json.parse 2-param) collided in IR lowering's
bare-name function table. fn_ast_map (name -> AST) was last-wins while
module.functions / resolveFuncByName are first-wins, so importing both and
calling one bound one function's AST against the other's FuncId and tripped
lazyLowerFunction's param-count assert (lower.zig:1606) — reached
unreachable code.

Fix:
- Register a namespaced import's OWN plain functions under their qualified
  name (ns.fn) in fn_ast_map, giving cli.parse / json.parse independent
  identities. The qualified resolution paths in CallResolver.plan /
  lowerCall already prefer ns.fn. NamespaceDecl now carries own_decls
  (populated in imports.addNamespace). Generic/comptime/pack/foreign
  functions are excluded (they dispatch by monomorphization off the bare
  template name); no eager declareFunction (it would resolve types before
  the forward-alias fixpoint).
- Make scanDecls' bare fn_ast_map registration first-wins so a later
  namespace recursion cannot clobber an earlier (flat) entry, aligning it
  with mergeFlat / resolveFuncByName.

Regression: examples/0719-modules-cli-and-json.sx imports both std.cli and
std.json under distinct namespaces and calls both parses; panics pre-fix,
passes after. issues/0100 marked RESOLVED.

src/ast.zig

@@ -674,6 +674,12 @@ pub const SpreadExpr = struct {
 pub const NamespaceDecl = struct {
     name: []const u8,
     decls: []const *Node,
+    /// Decls AUTHORED in the namespaced module itself (its `own_decls`), a
+    /// subset of `decls` (which also carries the module's transitive flat
+    /// imports). Lowering registers these under their module-qualified name
+    /// (`ns.fn`) so `pkg.fn(...)` resolves to a unique FuncId distinct from a
+    /// same-named function in another module (issue 0100).
+    own_decls: []const *Node = &.{},
     /// True when the namespace NAME was a backtick raw identifier — exempt
     /// from the reserved-type-name decl check (issue 0089).
     is_raw: bool = false,