refactor(sema): seal sema.zig as editor indexing only (A8.1)

Remove the last compiler dependency on sema as semantic truth and stop
publishing as-you-type sema diagnostics from the LSP.

- core.zig: drop dead `Compilation.analyze()`, the `sema_result` field,
  and the sema->diagnostics merge; drop the now-orphaned sema import.
  The CLI pipeline (parse -> resolveImports -> generateCode) never called
  analyze(), so this removes only dead code.
- lsp/server.zig: rename `analyzeAndPublish` -> `refreshEditorIndex` and
  delete its sema-diagnostic publish (and the now-unused `semaToLspDiags`).
  The editor index (doc.sema) is still refreshed for nav/refs/completion/
  tokens. On-save/on-open diagnostics still come solely from the canonical
  compiler pipeline in `runProjectCheck` (unchanged).
- Document sema as an editor-indexing API (doc.sema field comment).

Intended behavior change: as-you-type sema diagnostics no longer publish;
on-save canonical diagnostics are the sole source. CLI compile output and
the 361-example suite are unchanged (361/0, zero snapshot churn).

src/core.zig

@@ -2,7 +2,6 @@ const std = @import("std");
 const ast = @import("ast.zig");
 const parser = @import("parser.zig");
 const imports = @import("imports.zig");
-const sema = @import("sema.zig");
 const errors = @import("errors.zig");
 const c_import = @import("c_import.zig");
 const ir = @import("ir/ir.zig");
@@ -27,7 +26,6 @@ pub const Compilation = struct {
     import_sources: std.StringHashMap([:0]const u8),
     module_scopes: std.StringHashMap(std.StringHashMap(void)),
     import_graph: std.StringHashMap(std.StringHashMap(void)),
-    sema_result: ?sema.SemaResult = null,
     ir_emitter: ?ir.LLVMEmitter = null,
     /// Lowered IR module, kept alive past `generateCode` so post-link
     /// callbacks can re-enter the interpreter to invoke sx functions
@@ -128,18 +126,6 @@ pub const Compilation = struct {
         self.resolved_root = new_root;
     }
 
-    pub fn analyze(self: *Compilation) !void {
-        const root = self.resolved_root orelse self.root orelse return error.CompileError;
-        var analyzer = sema.Analyzer.init(self.allocator);
-        self.sema_result = analyzer.analyze(root) catch return error.CompileError;
-        // Merge sema diagnostics into our list
-        if (self.sema_result) |sr| {
-            for (sr.diagnostics) |d| {
-                self.diagnostics.add(d.level, d.message, d.span);
-            }
-        }
-    }
-
     /// Generate code via the IR pipeline: lower AST → IR → LLVM.
     pub fn generateCode(self: *Compilation) !void {
         // Heap-allocate the IR module so its address is stable during emit