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/install.sh

@@ -17,7 +17,8 @@
 # otherwise sudo prompts normally. The value is never stored in this repo.
 set -euo pipefail
 
-REPO="$(git -C "$(dirname "$(realpath "$0")")" rev-parse --show-toplevel 2>/dev/null || true)"
+HERE="$(dirname "$(realpath "$0")")"
+REPO="$(git -C "$HERE" rev-parse --show-toplevel 2>/dev/null || true)"
 REPO="${INSTALL_REPO:-$REPO}"
 [ -n "$REPO" ] && [ -d "$REPO/packages" ] || { echo "install.sh: no packages/ under ${REPO:-<repo>}" >&2; exit 1; }
 
@@ -36,9 +37,7 @@ SUDO() {
   else sudo "$@"; fi
 }
 
-read_manifest() { # $1 = manifest path; emits package names, strips comments/blanks
-  sed 's/#.*//' "$1" | tr -d '\r' | awk '{if ($1) print $1}'
-}
+source "$HERE/lib/manifest.sh"   # the same parser pkg-inventory.sh reads with
 # ---- desired-minus-installed delta ------------------------------------------
 PKGS=()
 for m in common.pkgs "$HOST.pkgs"; do