lsp: whole-program diagnostics from the real compiler on save

Reuse the compiler's lowering pass instead of re-implementing its checks
in sema. A module can't be lowered standalone — lowering only type-checks
functions reachable from a root — so the open file alone misses errors
like a *Move passed into a by-value method parameter. Drive the workspace
entry (main.sx) through parse → resolveImports → lowerToIR, then attribute
each diagnostic back to its file via source_file and publish per file
(clearing files whose errors are gone).

Runs on didOpen/didSave (disk-based); sema stays the live per-keystroke
layer. Advertise textDocumentSync.save so the editor sends didSave.

collectProjectDiagnostics is split out (transport-free) and covered by a
hermetic temp-project test.

src/lsp/types.zig

@@ -102,7 +102,7 @@ fn writeJsonValue(buf: *std.ArrayList(u8), allocator: std.mem.Allocator, value:
 /// Build the initialize result JSON.
 pub fn initializeResultJson(allocator: std.mem.Allocator) ![]const u8 {
     return std.fmt.allocPrint(allocator,
-        "{{\"capabilities\":{{\"textDocumentSync\":1,\"definitionProvider\":true,\"referencesProvider\":true,\"hoverProvider\":true,\"documentSymbolProvider\":true," ++
+        "{{\"capabilities\":{{\"textDocumentSync\":{{\"openClose\":true,\"change\":1,\"save\":true}},\"definitionProvider\":true,\"referencesProvider\":true,\"hoverProvider\":true,\"documentSymbolProvider\":true," ++
             "\"completionProvider\":{{\"triggerCharacters\":[\".\"]}}," ++
             "\"signatureHelpProvider\":{{\"triggerCharacters\":[\"(\",\",\"]}}," ++
             "\"semanticTokensProvider\":{{\"legend\":{{" ++