store the guid token in the root page for verification purposes.  and return it with the root page number as well.
prep for writing a block list for each bt segment.  written outside the segment itself and managed by the page manager.
after writing a bt segment, write a block list for it.  reserve the blocklist from the pagemanager before writing the bt root page, so it can contain a pointer to the blocklist.  have the page manager write the block list on the End() call.  block list is optional, decision made by the page manager.  returning 0 as the blocklist reserve ptr is fine.  fix the test suite to no longer assume the root page is always the last page of a file, for situations where the test suite is still writing every bt segment into its own file.  which will remain legal.  in fact, I could have left the lastPage() assumption in place by just always using a page manager which doesn't write block lists (in those one-bt-segment-per-file cases).
use List.isEmpty instead of comparing list length against 0.  perf.
terminology change for pagebuilder, write instead of flush
fixed a few naming change places I missed
give the two folder functions different names
typo
undo the blocklist stuff.  storing one blocklist per page is really wasteful when the pagesize is large.  gotta make this more efficient.  and storing the blocklist inside the btree segment is tricky because storing the blocklist can change the blocklist.
notes and initial code for dealing with saving segment list and block lists for segments