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

tests/manifest-parser.bats

@@ -0,0 +1,51 @@
+#!/usr/bin/env bats
+# tests/manifest-parser.bats — the one manifest parser, shared by install.sh and pkg-inventory.sh
+#
+# `packages/*.pkgs` is hand-written, so the parser's whole job is to forgive the ways a hand-edited
+# list is wrong. The same function decides what install.sh tries to install and what pkg-inventory
+# calls drift, so a manifest line must mean the same thing to both — which is why it lives in one
+# file instead of being pasted into each caller (Refs #60: it was pasted twice).
+#
+# Observed through the function's exit status and stdout only:
+#   names        first field of a line, one bare token; `#` and everything after it is a comment
+#   CRLF         tolerated (the manifests get edited on more than one box)
+#   blank lines  vanish
+#   unreadable   non-zero with no names — an empty desired list must never be a silent success,
+#                because "nothing desired" is exactly what a rebuild would install
+
+setup() {
+  TEST_TMP="$(mktemp -d)"
+  LIB="$BATS_TEST_DIRNAME/../scripts/lib/manifest.sh"
+}
+
+teardown() { rm -rf "$TEST_TMP"; }
+
+parse() { # $1 = manifest contents, verbatim
+  printf '%s' "$1" > "$TEST_TMP/m.pkgs"
+  run bash -c 'source "$1"; read_manifest "$2"' _ "$LIB" "$TEST_TMP/m.pkgs"
+}
+
+@test "inline notes, full-line comments, blank rows and CRLF come out as bare names" {
+  parse $'# desired state\r\nghostty          # terminal\r\n\r\n   \r\nzoxide\r\n'
+  [ "$status" -eq 0 ]
+  [ "$(printf '%s\n' "$output" | tr '\n' ' ')" = "ghostty zoxide " ]
+}
+
+@test "only the first field of a line is a package" {
+  # install.sh hands every emitted token straight to pacman, so a stray second token on a line
+  # must never become an install target; whatever it is, the manifest did not declare it.
+  parse $'neovim extra tokens here\n'
+  [ "$output" = "neovim" ]
+}
+
+@test "a comment that names a package is not a package" {
+  parse $'# vlc\n#typora\nbun\n'
+  [ "$output" = "bun" ]
+}
+
+@test "an unreadable manifest fails loudly instead of yielding an empty desired list" {
+  run --separate-stderr bash -c 'source "$1"; read_manifest "$2"' _ "$LIB" "$TEST_TMP/nope.pkgs"
+  [ "$status" -ne 0 ]
+  [ -z "$output" ]
+  [[ "$stderr" == *nope.pkgs* ]]
+}