image digest: single read, cache file read & hash

For image digests cache file read & hash, one read per image
file

Fix issue: images referenced from several objects were read and
hashed once per reference.

  [as an example in the sisu-manual sm_tux.png accounts for 6 of
  the document's image records (12 across the sample set), each
  of them a fresh read of the same bytes, and source_pod.d hashes
  the file again for digests.txt.]

Memoise on the full path within safeComputeHashAndSize. static is
thread-local in D, so documents processed in parallel each get
their own cache and no synchronisation is involved. Files are not
expected to change during a run.

Output is unchanged: identical .ssp output. (An image that is
missing or unreadable now warns once to stdout rather than once
per reference).

(assisted by Claude-Code)

0 files changed, 0 insertions, 0 deletions