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/types.zig

@@ -2,6 +2,12 @@ const std = @import("std");
 const ast = @import("ast.zig");
 const Node = ast.Node;
 
+/// Editor metadata type model, used only by `src/sema.zig` (the language-server
+/// symbol/type index) for navigation, completion, and hover. NOT the compiler's
+/// source of truth: lowering, codegen, and layout use the canonical
+/// `TypeId` / `TypeTable` model in `src/ir/types.zig`. Do not expand this to
+/// carry new compiler semantics; the architecture endpoint (phase A8) is to
+/// delete it or reduce it to display-only data derived from `TypeId`.
 pub const Type = union(enum) {
     // Variable-width integers (1–64 bits)
     signed: u8,