v0.15.0

Kept paths carried across runs (browser-substrate ADR-0152, S2 AC-LC10). Additive: two new exports, which a host
detects by their presence before it hands a seed, so an engine without them is never asked; nothing a realm resolves
changes for a host that hands none, and no lowered text changes, so the prepared format stays tabnode-prepared-7.
Cut as 0.15.0, a minor release, for the new public exports.

• A process can start with the paths its project's last run kept (browser-substrate ADR-0152, S2 AC-LC10).
  seedKeptResolutions(entries, filesystem?) seeds every kept-paths table the realm makes (only that filesystem's, where one is named) with (directory + "\0" + name, path)
  pairs, within R76's bounds; keptResolutions() returns what the realm's tables keep now. The host hands a seed only
  for a table resolved on the same files; nothing changes for a host that hands none. resolveSplit.keptPaths.carried
  says how many a table started with.