installer: stop newest_matching_file from dying on SIGPIPE

newest_matching_file() piped `ls -1t` into `head -1`. Under `set -o pipefail`
head closes the pipe after the first line, ls dies on SIGPIPE, and the function
returns 141 having printed nothing. Its callers assign it at top level under
`set -e`, so that 141 aborts the install.

It takes roughly 1600 matching names to fill the pipe buffer, which is why this
has sat unnoticed: with two or three files the old code is correct. Measured on
origin/next with 5001 matches, the function returns 141 and prints nothing; with
this change it returns rc=0 and the right filename.

Two of the four callers are the "find the newest .mosaic-bak-* backup" lookup,
which is the path a restore leans on.

Reading the listing into an array through process substitution has no pipeline,
so there is nothing for pipefail to catch. This also clears the one remaining
violation `scripts/pipefail-early-exit.test.mjs` reports against tools/install.sh
-- that test lives on main, not on next, so it starts failing the moment main is
merged into next for the 0.0.50 integration.

tools/install-newest-matching-file.test.sh pins it, including the large-population
case that is the whole point. Red on origin/next (rc=141), green here.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WYgWocp36goy8hj2ui6ps1

tools/install.sh

@@ -169,8 +169,16 @@ newest_matching_file() {
   matches=("$dir"/$pattern)
   shopt -u nullglob
   [[ "${#matches[@]}" -gt 0 ]] || return 0
+  # Read the whole listing and take the first entry, rather than piping into
+  # `head -1`. Under `set -o pipefail`, head closes the pipe after one line, ls
+  # dies on SIGPIPE, and the function returns 141 -- so on the day the directory
+  # holds enough files to fill a pipe buffer, finding the newest one starts
+  # failing the install. Process substitution has no pipeline to fail.
+  local -a sorted=()
   # shellcheck disable=SC2012 # Need portable mtime sorting across Linux/macOS.
-  ls -1t "${matches[@]}" 2>/dev/null | head -1
+  mapfile -t sorted < <(ls -1t "${matches[@]}" 2>/dev/null)
+  [[ "${#sorted[@]}" -gt 0 ]] || return 0
+  printf '%s\n' "${sorted[0]}"
 }
 
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