chore: initialize project scaffold

Add .gitignore (excludes .claude/, venv, checkpoints, processed data,
external corpora), .gitattributes (LF normalization, binary markers),
full directory tree with .gitkeep placeholders, and src __init__ stubs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,32 @@
+# Normalize line endings to LF on commit (cross-platform safety)
+* text=auto eol=lf
+
+# Python source
+*.py text eol=lf
+
+# Custom text formats
+*.chord text eol=lf
+*.md text eol=lf
+*.txt text eol=lf
+*.csv text eol=lf
+*.json text eol=lf
+*.yaml text eol=lf
+*.yml text eol=lf
+*.toml text eol=lf
+*.cfg text eol=lf
+*.ini text eol=lf
+
+# Binary assets — never diff/merge
+*.pt binary
+*.pth binary
+*.ckpt binary
+*.pkl binary
+*.mid binary
+*.midi binary
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.pdf binary
+*.zip binary
+*.gz binary
+*.tar binary