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}"

tests-impl/test-compute-version.bats

@@ -0,0 +1,58 @@
+#!/usr/bin/env bats
+
+# Tests for scripts/compute-version.sh
+# Invokes the real script against the real repo state.
+
+setup() {
+  REPO_ROOT="$(git rev-parse --show-toplevel)"
+  SCRIPT="$REPO_ROOT/scripts/compute-version.sh"
+}
+
+@test "script exists and is executable" {
+  [ -x "$SCRIPT" ]
+}
+
+@test "output matches '<upstream-tag>-seth<N>' format" {
+  run "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [[ "$output" =~ ^[0-9].+-seth[0-9]+$ ]]
+}
+
+@test "N=1 when no prior seth-tags exist" {
+  # This test assumes a clean tag db. If there ARE existing seth-tags, this
+  # test will report N>1 and that's the correct value. Skipped if any
+  # upstream-tag-seth* tag already exists.
+  upstream_tag=$(git describe --tags --abbrev=0 upstream/master)
+  existing=$(git tag --list "${upstream_tag}-seth*" | wc -l)
+  if [ "$existing" -gt 0 ]; then
+    skip "seth-tags already exist (count=$existing); N=1 invariant only holds on first release"
+  fi
+  run "$SCRIPT"
+  [ "$status" -eq 0 ]
+  [[ "$output" == "${upstream_tag}-seth1" ]]
+}
+
+@test "N increments past existing seth-tags" {
+  upstream_tag=$(git describe --tags --abbrev=0 upstream/master)
+  # Create a fake seth-tag for this test, then clean up.
+  fake_tag="${upstream_tag}-seth99"
+  git tag "$fake_tag" 2>/dev/null || true
+  run "$SCRIPT"
+  git tag -d "$fake_tag" >/dev/null 2>&1 || true
+  [ "$status" -eq 0 ]
+  # Existing count was at least 1 (our fake), so N >= 2.
+  n="${output##*-seth}"
+  [ "$n" -ge 2 ]
+}
+
+@test "fails cleanly when upstream/master ref is missing" {
+  # Run in a temp git repo with no upstream remote.
+  tmp=$(mktemp -d)
+  cd "$tmp"
+  git init -q
+  git commit --allow-empty -m "init" -q
+  run "$SCRIPT"
+  cd "$REPO_ROOT"
+  rm -rf "$tmp"
+  [ "$status" -ne 0 ]
+}