refactor(packages): one manifest parser, so a name means one thing to both tools

install.sh and pkg-inventory.sh carried byte-identical copies of read_manifest.
That is the duplicated-code half of the problem; the worse half is untested —
neither script had any test for the grammar that decides what gets installed.

- scripts/lib/manifest.sh is the parser; both scripts source it
- tests/manifest-parser.bats writes the contract first (comments, CRLF, first
  field only, unreadable file fails loudly instead of yielding "nothing desired")
- an unreadable manifest now fails inside the helper rather than depending on
  whichever `set` options the caller happens to run under
- install.sh check mode verified end to end against a stubbed pacman: a CRLF
  manifest with trailing comments resolves to bare names, installed names drop

Refs #60

scripts/lib/manifest.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# scripts/lib/manifest.sh — the one parser for packages/*.pkgs
+#
+# install.sh walks desired → installed; pkg-inventory.sh walks installed → declared. Both read the
+# same hand-written manifests, so both must agree on what a line MEANS: if one forgives an inline
+# comment and the other does not, the same name is simultaneously "desired" and "drift", and the
+# pair stops being a check on the machine. Source it — do not paste the body into a caller again
+# (Refs #60: it was pasted twice, in install.sh and in pkg-inventory.sh).
+#
+# Contract, and tests/manifest-parser.bats is that contract:
+#   name         first field of a line, emitted as one bare token — callers hand it to pacman
+#   comment      `#` and everything after it is dropped, whole-line or trailing
+#   CRLF         stripped (the manifests are edited on more than one box)
+#   blank        empty and whitespace-only rows vanish
+#   unreadable   non-zero exit, no names: a silent empty list reads as "nothing desired", and a
+#                rebuild would then install nothing while reporting success
+#
+# Sourced, never executed; callers own `set -euo pipefail`.
+
+read_manifest() { # $1 = manifest path; prints one package name per line
+  [[ -r "$1" ]] || { printf 'read_manifest: cannot read %s\n' "$1" >&2; return 2; }
+  sed 's/#.*//' "$1" | tr -d '\r' | awk '{if ($1) print $1}'
+}