ocda: fixed inverted object_number_off on body paragraphs

ssp: make use of curly braces (code-style)

(assisted by Claude-Code)

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)

Complete image file digest identity record: add
px:<width>x<height> to the .ssp image record, read from the
image file header (imageformats.read_image_info, no decode):

  .image: won_benkler_2_1.png sha256:5BA4...D128 bytes:93861 px:420x342

(The w##h## dimensions in the markup are display dimensions, not the
file's: they are author set, or where given as w0h0 are derived by
_image_dimensions and capped to a maximum display width, and they
become the width/height attributes of the output format image display).

(assisted by Claude-Code)

image digest: record missing or unreadable image, do not abort

safeComputeHashAndSize() open image with guard, so a document
naming an image absent from media/image/ throws
ErrnoException out of the abstraction stage.

as with _image_dimensions() (which warns and continues where issue
with image) warn to stdout and record the fact in the abstraction:

  .image: ffa.png missing:true

the abstraction would now state that the object embeds an image it
could not read, rather than the .ssp resemble one produced from a
complete pod. The sha256 summary line marks the same case
MISSING OR UNREADABLE.

(assisted by Claude-Code)

a repeatable record, one line per image, filename first:

  .image: won_benkler_2_1.png sha256:5BA492C2...D128 bytes:93861

an object embedding more than one image emits more than one .image
line.

(assisted by Claude-Code)

Modules and imports rewritten to sisudoc.ocda.* and
sisudoc.outputs.*; dub.json excludedSourceFiles and the
spine:abstraction sub-package sourcePaths collapsed to
./src/sisudoc/ocda.

Verified: nix build .#spine-overlay-ldc clean.

(assisted by Claude-Code)

create new directories under ./src/sisudoc ocda & outputs in order to
separate the document abstraction library from downstream output
processing (stuff broken till paths & modules fixed)