feat(stdlib/S1.1): DeclId for every declaration — additive DeclTable [additive]

Build a DeclTable in parallel with the import facts: every RawDeclRef
(source / imported / namespaced / C-imported) gets a stable DeclId carrying
source path, display name, AST node identity, span, and DeclKind. Namespace
targets record their members' DeclIds (NamespaceTarget.member_ids). A generic
struct's template is keyed by DeclId in a parallel struct_template_by_decl
store, written alongside the live name-keyed struct_template_map.

A Debug-only round-trip cross-check (RawDeclRef -> DeclId -> AST node ptr)
asserts the table identifies the same node across the corpus, run from
buildDeclTable and pinned by a unit test.

Additive (S0.1 class: mirror): the old maps stay active and lowering still
consumes them; nothing reads the DeclTable / struct_template_by_decl for
selection yet (the S4 cutover does). Generated IR + output bytes are unchanged
by construction.

Gate over the baseline-green corpus: zig build, zig build test (424/424),
bash tests/run_examples.sh (540 passed) — all exit 0; single-author output
byte-identical (37 .ir snapshots unchanged).

src/ir/lower.zig

@@ -15487,6 +15487,17 @@ pub const Lowering = struct {
             const tmpl = self.buildGenericStructTemplate(sd, source_file) orelse return;
             self.program_index.struct_template_map.put(tmpl.name, tmpl) catch {};
 
+            // S1.1 (additive): key the template by DeclId in parallel. Nothing
+            // reads this for selection yet; `struct_template_map` stays the live
+            // consumer. A template whose decl is not in the table (comptime /
+            // block-local registration with facts unwired) keeps only the
+            // name-keyed entry.
+            if (self.program_index.decl_table) |dt| {
+                if (dt.declIdForStructDecl(sd)) |id| {
+                    self.program_index.struct_template_by_decl.put(id, tmpl) catch {};
+                }
+            }
+
             // Register methods under "TemplateName.method" in fn_ast_map
             for (sd.methods) |method_node| {
                 if (method_node.data == .fn_decl) {