An include or import cycle recursed without bound and crashed the
host with a stack overflow, reachable from stylesheet data. The
compile chain tracks the active load chain by resolved href and drops
re-entry, while diamond composition keeps loading both copies with
their own import precedences per 2.6.2. id() builds a per-document
index on first use instead of re-walking the document per call,
matching key(). Cycle and diamond regression tests added.
Mihaela Mihaljevic
6c0afa3749