lm15 1.1.2 for R

Implements the lm15 contract at commit 8eedbae, the same commit as lm15 for
Python 1.2.3, TypeScript and Rust 1.0.0-rc.7 and Go v1.1.0-rc.6, and passes
all 1,904 of its checks.

• response_text() and parse_json() read a structured answer that came
  back as a data part (a JSON schema with a boolean or enum property is a
  judgment request, answered that way); they returned NULL and failed.

• reasoning(thinking_budget = 1024) works without effort: it is the
  highest level whose budget (1024, 2048, 8192, 16384, 24576, 32768) is at or
  below the given one, "minimal" below 1024. reasoning("none") and a zero
  budget say how to turn thinking off (reasoning("off")).