feat(scriberr): overlap-and-stitch Parakeet slicer patch, rebuild script, bench

Carry patches/0001 on our Scriberr build (upstream a353078): adjacent
buffered chunks overlap by 4 s inside --chunk-len and hand over at a word
both chunks transcribed alike, instead of cutting at fixed marks with no
overlap. Pause-aware cutting is included as an opt-in (--pause-search);
it measured neutral once the stitch was right. The Go<->Python CLI and
JSON seam is unchanged.

Bench (4 recordings, 118 min, 3 cut placements each, against a no-cut
whole-file reference; metrics only, private audio stays on fv-ml1):
cuts with an error within +-3 s fall from 52% (93/179) to 22% (41/184)
against a 19% background; floor +-0.08. Positive control: upstream's
cutter +0.33 over background. A-vs-A byte-identical in-process and
across CLI processes. Peak GPU memory unchanged at 5,496 MiB (n=3).
Also found: Parakeet skips runs of >=10 words mid-chunk with any
slicer, upstream's included; not addressed here.

scripts/scriberr-rebuild clones a pinned upstream sha into a new
/opt/docker/src dir, git-apply-checks the patches, builds a distinct
tag, and checks embed, unit tests, the JSON seam (scriberr-seam-check.py)
and the memory budget on idle GPU 3. Deploy stays manual. The upstream
PR is prepared under patches/upstream-pr/ and not opened.

stacks/scriberr/.env.example

@@ -4,6 +4,11 @@
 # ── Image ────────────────────────────────────────────────────────────────
 # Built locally from Dockerfile.cuda.12.9 — see the compose header for why
 # the published scriberr-cuda image is NOT usable on these Blackwell cards.
+# Patched builds come from scripts/scriberr-rebuild and are tagged
+#   scriberr:local-blackwell-<upstream sha7>-<suffix>   (e.g. -a353078-slicer1)
+# so every build keeps its own tag. Deploying = pointing this at a new tag;
+# rollback = pointing it back. See stacks/scriberr/patches/README.md.
+# (Unset falls back to the original unpatched scriberr:local-blackwell.)
 SCRIBERR_IMAGE=scriberr:local-blackwell
 
 # ── Network ──────────────────────────────────────────────────────────────