fix(portability): replace bash 4 builtins absent from macOS bash 3.2

macOS ships bash 3.2 as /bin/bash, which `#!/usr/bin/env bash` resolves to
when no newer bash is on PATH. Two builtins the repo relies on do not exist
there, and both failed SILENTLY:

- `mapfile` in the three surgical-commit helpers left every array empty, so
  the scope guards passed on nothing (fail-OPEN) and the commits degraded to
  "nothing pending — no-op" while reporting success. deploy-commit.test.sh
  went 4/16; memory and doc commits simply never happened.
- `declare -A` in the session-start hook errored on every session and left
  each plugin cost at 0, so the passive-budget warning could never fire.

`_read_lines_into` is the portable equivalent of `mapfile`, space-safe and
resetting its target first — expanding a never-assigned array trips `set -u`
on bash < 4.4, which is how the empty arrays surfaced as "unbound variable".
Plugin costs move to a `case`.

source-scope.sh's header prescribed `mapfile` to its callers; it now shows
the read loop, and its own test plus run-reconcile.sh stop using the builtin.

deploy-commit 16/16, source-scope 34/34, run-reconcile 25 GREEN / 0 RED,
session-start stderr empty.

lib/tests/run-reconcile.sh

@@ -21,7 +21,10 @@ has() { printf '%s\n' "$1" | $GREP -qF -- "$2"; }   # substring present in multi
 
 echo "=== T1 recursive coherence — enumerate from BODY, never the ## Index ==="
 DRIFT="$FIX/registry-index-drift.md"
-mapfile -t IDS < <(reconcile_enumerate_ids "$DRIFT" LRN)
+# bash 3.2 (macOS /bin/bash) has no mapfile; read the ids explicitly. IDS is
+# seeded empty so `set -u` cannot trip on an unset array below.
+IDS=(); while IFS= read -r _id; do IDS+=("$_id"); done \
+  < <(reconcile_enumerate_ids "$DRIFT" LRN)
 if [ "${#IDS[@]}" -eq 72 ]; then ok "T1a enumerated 72 body ids"; else no "T1a got ${#IDS[@]}, expected 72 (an Index-reader gives 51)"; fi
 if printf '%s\n' "${IDS[@]}" | $GREP -qx "LRN-020"; then ok "T1b includes body-only canary LRN-020"; else no "T1b dropped LRN-020 — read the Index, not the body"; fi
 # teeth: an Index-based enumerator would RED here (the fixture discriminates)