feat: add compute-version.sh + bats tests

Emits <upstream-tag>-seth<N> to stdout. Pure logic, no side effects.
5/5 bats tests pass; output verified as 3.99-master618-seth1.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/compute-version.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+# Emit "<upstream-tag>-seth<N>" version string to stdout.
+# Pure logic: no side effects.
+#
+# CALLER RESPONSIBILITY (per spec §5.4): the local tag db must be fresh.
+# If invoked outside the release flow, run `git fetch origin --tags` first
+# or risk a stale <N> value.
+#
+# Spec: sethlabels-docs/specs/2026-04-29-packaging-design.md §5.4 (D4)
+set -euo pipefail
+
+upstream_tag=$(git describe --tags --abbrev=0 upstream/master)
+existing_count=$(git tag --list "${upstream_tag}-seth*" | wc -l | tr -d ' ')
+next_n=$((existing_count + 1))
+echo "${upstream_tag}-seth${next_n}"