test: one disc resolver, no machine-specific defaults, all three corpora in the container

Finishes #16 in the three places its earlier remedies missed.

`tests/`: the last four local `disc_root()` copies now use `tests/common`, and
with them goes the one real hardcoded fallback — `ui_keyframe_record_disc.rs`
fell back to an absolute path on one machine, which made `unset SYLPHEED_DISC`
a no-op there. Control: with the corpus absent that suite now finishes in 0.00s
instead of 57.55s, so it skips rather than finding a disc of its own.

`examples/`: seventeen examples defaulted to `/disc`, the mount point inside the
CI container. Redundant there — `docker/ci/run` sets `SYLPHEED_DISC=/disc` — and
wrong everywhere else, where a missing corpus turned into a file-not-found
against a path that has never existed on the host. They now name the variable to
set, like the other hundred examples already did.

`docker/ci/run`: mount `$SYLPHEED_RES3D` and `$SYLPHEED_ISO` alongside the disc.
Only the disc was mounted, so an in-container run sat out the res3d and iso
suites while looking like a full one — the defect this issue is about, in the
runner itself.

Measured in the container on this desktop with all three corpora present:
45 suites / 377 passed / 0 failed / 14 ignored, and `sylpheed-corpus-report.txt`
now reports PRESENT for all three rather than for the disc alone.

Refs #16.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docker/ci/run

@@ -36,8 +36,20 @@ args=(
   -w /work
 )
 
-# The disc, read-only, when a disc-backed test or the exporter needs it.
+# The corpora, read-only, when a disc-backed test or the exporter needs them.
+#
+# All three, not just the disc: a suite whose corpus is absent self-skips and
+# still counts as passed, so mounting one of three made an in-container run look
+# like a full one while `res3d` and `iso` suites silently sat out (#16). Each is
+# mounted only when it exists, and `target/sylpheed-corpus-report.txt` says which
+# ones the run actually had.
 DISC="${SYLPHEED_DISC:-$REPO/../sylph_extract}"
 [ -d "$DISC" ] && args+=(-v "$DISC:/disc:ro" -e SYLPHEED_DISC=/disc)
 
+RES3D="${SYLPHEED_RES3D:-}"
+[ -n "$RES3D" ] && [ -d "$RES3D" ] && args+=(-v "$RES3D:/res3d:ro" -e SYLPHEED_RES3D=/res3d)
+
+ISO="${SYLPHEED_ISO:-}"
+[ -n "$ISO" ] && [ -f "$ISO" ] && args+=(-v "$ISO:/disc.iso:ro" -e SYLPHEED_ISO=/disc.iso)
+
 exec docker run "${args[@]}" "$IMAGE" "$@"