refactor(ir): extract ProgramIndex, move low-fanout decl facts (A1.1a)

Architecture phase A1.1a. Introduce src/ir/program_index.zig as the single
storage owner for declaration-name / import / visibility facts, and move the
three low-fanout maps out of the Lowering state bag:

- import_flags     (owned by ProgramIndex)
- module_scopes    (borrowed pointer into a core.zig-owned map)
- import_graph     (borrowed pointer into a core.zig-owned map)

Lowering embeds one ProgramIndex by value and reaches every moved fact through
self.program_index.<field>; later phases hand collaborator modules a
*ProgramIndex instead of *Lowering. 8 call sites in lower.zig + 2 setters in
core.zig repointed. No duplicate storage, no fallback path; zig build enforces
no missed reference.

Mutation-heavy registration (registerStructDecl etc.) stays in Lowering and
now writes import_flags through the index. High-fanout maps are deferred to
A1.1b.

Adds src/ir/program_index.test.zig (init-empty, import_flags round-trip,
borrowed-view ownership) wired into the ir.zig barrel.

Behavior-preserving: zig build, zig build test, and bash tests/run_examples.sh
(350 passed, 0 failed) all green.

src/core.zig

@@ -288,8 +288,8 @@ pub const Compilation = struct {
         lowering.resolved_root = root;
         lowering.target_config = self.target_config;
         lowering.diagnostics = &self.diagnostics;
-        lowering.module_scopes = &self.module_scopes;
-        lowering.import_graph = &self.import_graph;
+        lowering.program_index.module_scopes = &self.module_scopes;
+        lowering.program_index.import_graph = &self.import_graph;
         lowering.lowerRoot(root);
         if (self.diagnostics.hasErrors()) return error.CompileError;