docs(ir): mark sema/types as editor-only, not compiler truth (A1.2)

Architecture phase A1.2 — documentation/comment only, no behavior change.

Resolve the ambiguity over which type model compiler decisions trust:

- src/sema.zig: file-level module doc stating it is the editor symbol/type
  index for the language server (navigation/completion), NOT a compiler
  semantic pass. Its Type values are editor metadata; the compiler uses the
  canonical TypeId/TypeTable model in src/ir/. sx requires no as-you-type type
  checking -- authoritative diagnostics are produced on save by the canonical
  pipeline. Added notes on SemaResult, Analyzer, resolveTypeNode, inferExprType.
  No public API renamed (would churn LSP call sites).
- src/types.zig: note that Type is editor metadata only, not compiler truth;
  do not expand for new compiler semantics (A8 deletes/reduces it).
- src/ir/types.zig: fix stale TypeTable.aliases comment -- it borrows
  Lowering.program_index.type_alias_map (post-A1.1b).

Deleting the LSP's parallel sema diagnostic stream is A8.1, not this step.

Gate green: zig build, zig build test, bash tests/run_examples.sh (350 passed).

src/ir/types.zig

@@ -332,7 +332,7 @@ pub const TypeTable = struct {
     slice_arena: std.heap.ArenaAllocator,
     /// Target pointer size in bytes (4 for wasm32, 8 for 64-bit targets).
     pointer_size: u8 = 8,
-    /// Borrowed pointer to Lowering's `type_alias_map`. When set,
+    /// Borrowed pointer to `Lowering.program_index.type_alias_map`. When set,
     /// `resolveTypeName` consults it before falling through to
     /// the empty-struct-stub default — so a name like `ShaderHandle`
     /// (defined `ShaderHandle :: u32`) resolves to `u32` rather than