Migrate to Zig 0.16.0

Simplify production code: deduplicate, remove dead code, consolidate …

…patterns

- Extract sendRawRequest in LspClient to deduplicate initialize/sendRequest
- Registry.register derives key from definition.name (removes duplicate param)
- Replace makeProps with mcp_types.makeProperty (removes duplicate function)
- Replace inline DidOpenParams/CloseParams with lsp_types equivalents
- Remove DocInfo.uri redundant field, use map key directly
- Consolidate file:// prefix stripping into uri_util.stripFilePrefix
- Remove trivial wrappers: runCommand, formatLocationsResponse
- McpTransport.readMessage uses self.stdin_file instead of hardcoded stdin
- Delete unused cmd/zig_runner.zig and cmd/zvm.zig

Net result: -175 lines, 104 tests still passing.