ichor(D2): fix package docstrings so it compiles; verify green

Pony allows one package docstring per package — collapse the four secondary
files' leading docstrings to // comments. ichor now compiles on ponyc 0.64.0
and the smoke wiring runs green (internal perfused, external rejected at D1,
cross-organ delivered). Update README + D2 status to reflect it builds; ignore
build/.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015hmgREHNsxYCuim33yUF2c

.gitignore

@@ -0,0 +1,3 @@
+
+# Pony build output (ichor)
+build/