Add README.md; fix second stale main.cpp reference in CLAUDE.md

README.md: dependencies, build instructions for all three targets
(verified all six PRJ x CONFIG combinations build cleanly from a fresh
make clean, plus make clean's CONFIG scoping, default invocation,
incremental no-op rebuilds, and the install target), how to run each
binary, GPU acceleration usage, and a project-files overview.

CLAUDE.md's "Entry points" section still described main.cpp's old
CREATE_TEST/TRAIN_TEST behavior -- missed this on the earlier pass that
fixed the "Build" section's description of the same file.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016K8Gu7Qejd11JbdiHZqYAs

CLAUDE.md

@@ -92,8 +92,8 @@ behavior change, same output, just optionally GPU-accelerated matrix multiplies.
 
 ### Entry points
 
-- **`source/main.cpp`** — `TEST` target; exercises `DeepStack` directly (build/train/save a small DBN,
-  or load an existing one, depending on `CREATE_TEST`/`TRAIN_TEST` compile-time flags).
+- **`source/main.cpp`** — `TEST` target; the project test suite (see "Build" above) — loads each named
+  project under `prj/<name>/` and checks its reconstruction error.
 - **`source/poet.cpp`** — `POET` target; loads an `RnnStack` project (default `poet_2v_5s`) and dispatches
   on `argv[1]`: `c`(reate)/`r`(eset weights)/`t`(rain, optionally with a training-text path in `argv[2]`)/
   `f`(orward-generate text, optional seed string in `argv[2]`). Training periodically calls back into