Import CXXR runtime source snapshot

[isomorphisms]

Imports the final upstream CXXR/Rho source tree under cxxr/ for side-by-side study with ir's active R source in code/.

• upstream: rho-devel/rho
• upstream commit: bf6c652b826cd892e88630eb35f182b614af52b1
• upstream source tree: b12c08f052d89467122a0deee381f13609ea4e40
• snapshot is unmodified and retains its GPL-2 license
• no current ir build path compiles or links cxxr/

Verification:

• copied tree compared byte-for-byte with the upstream checkout
• all 9,072 CXXR files are present, including the symlink and files ignored by upstream ignore rules
• cxxr/configure --help executes successfully
• the published commit tree exactly matches the verified local commit tree: 071b036e442236a0cafbf3746bf2f4b645aa8cf7