docs: archive the frozen stage code as one compressed tarball

docs/history kept six stages with a full copy of the project under code/,
3.8 mb of sources duplicated from stage to stage and already superseded. store
them as docs/history/code-snapshots.tar.gz, 0.66 mb, and extract one in place
to read it. every stage's notes.md stays browsable, and the index says where the
code went and how to get it back.

the top-level README points at the archive too. its other two links into
docs/history, 21-micro and 35-bench, were notes that were never kept and do not
resolve; the index now records that.

1 files changed, 0 insertions, 2 deletions

diff --git a/docs/history/17-o3-boundaries/code/tests/borrowmove.zc b/docs/history/17-o3-boundaries/code/tests/borrowmove.zc
deleted file mode 100644
index 43f27b2..0000000
--- a/docs/history/17-o3-boundaries/code/tests/borrowmove.zc
+++ /dev/null
@@ -1,2 +0,0 @@
-fn put(.m box, int v) { *int c = new(int); *c = v; box[0] = c; return 0; }
-fn main() { let b = new(1); put(b, 6); putn(*(b[0] as *int)); putc(10); return 0; }