🧪 TEST: add a Sphinx-Needs kitchen-sink bundle to the example

[ubmarco]

Blocked on useblocks/sphinx-needs#1750.
The PlantUML !include in this bundle needs that fix. The testing group
therefore points at sphinx-needs master for now — CI's pytest/bazel jobs go
green as soon as #1750 lands (tox-uv resolves the git ref fresh, so no
uv.lock refresh is needed), and the pin is then swapped for
sphinx-needs>8.3.0 once that version ships.

What and why

The showcase/ folder has one bundle per file-referencing directive. Sphinx-Needs
did not fit that shape: it contributes three doc-relative file references, and
they do not behave alike.

reference	resolved by	Sphinx dependency	seen by path_check
needimport:: needs.json	Sphinx relfn2path	yes	yes
needreport :template:	Sphinx relfn2path	no	no
needuml / needarch !include	the PlantUML process	no	no

So showcase/needs/ is a deliberate kitchen sink — one page per directive, each
sitting next to the file it reads — because the three only make sense side by
side.

The middle column is the finding worth surfacing. path_check works off
env.dependencies, which is exactly why it is directive-agnostic — and exactly
why it cannot see two of these three. docs/source/configuration.rst now says so
outright, instead of leaving readers to assume the check is total.

Also in here

• First use of exclude in the example. report-template.rst is Jinja
  input, not a document, but carries the conventional .rst suffix — so a
  directory mount walks it and publishes an orphan page of unrendered Jinja. The
  bundle's .puml / .json files need no exclude; their suffixes are not in
  source_suffix.
• The shared-ubproject.toml convention, demonstrated. The host sets
  needs_from_toml = "ubproject.toml", so Sphinx-Needs reads [needs] from the
  same file whose [[mounts]] array sphinx-mounts reads. The README has
  advertised this; now the example does it.
• plantuml_output_format = "svg_img". Sphinx-Needs stamps a scale
  attribute (100 — a no-op) onto every diagram node it generates, and
  sphinxcontrib-plantuml's PNG path warns about any scaling attribute unless
  Pillow is installed, which fails this -nW build over nothing. The SVG path
  does no scaling, so this avoids a dependency rather than adding one.

Tests

Both layers, so this is not gated behind the Bazel job:

• tests/test_path_directives.py — three new tests next to the existing
  per-directive ones. needimport and needreport stay renderer-independent
  like their neighbours; the needuml !include test has to render for real
  (PlantUML resolves that path itself and records nothing Sphinx can assert on),
  so it skips without java/plantuml.
• tests/test_example.py — asserts each page rendered, the imported needs became
  real needs, the report template was applied, the excluded template published no
  page, and that the bundle-local .puml reached both rendered SVGs.

Verified locally against the patched sphinx-needs: 168 passed (was 163), docs
build clean under -nW, prek run --all-files passes.

Verified in the other direction too — against unpatched sphinx-needs the new
needuml test and the example build both fail with
plantuml command 'plantuml' cannot be run, which is the #1749 symptom. The
showcase genuinely exercises the fix rather than merely coexisting with it.