`wrap` layer expansion can loop forever when the core content contains `{CORE_TEMPLATE}`

[pkoppularipple]

Affected: scripts/bash/common.sh:606, confirmed present at tag v0.11.9
(fetched from raw.githubusercontent.com/github/spec-kit/v0.11.9/scripts/bash/common.sh).

The wrap strategy substitutes the core content into the layer at each {CORE_TEMPLATE}
placeholder:

case "$layer_content" in
    *'{CORE_TEMPLATE}'*) ;;
    *) echo "Error: wrap strategy missing {CORE_TEMPLATE} placeholder" >&2; return 1 ;;
esac
while [[ "$layer_content" == *'{CORE_TEMPLATE}'* ]]; do
    local before="${layer_content%%\{CORE_TEMPLATE\}*}"
    local after="${layer_content#*\{CORE_TEMPLATE\}}"
    layer_content="${before}${content}${after}"
done

The loop condition re-tests the string it just substituted into. If $content itself
contains the literal {CORE_TEMPLATE}, every iteration reintroduces the placeholder, the
condition never goes false, and layer_content grows by ${#content} each pass — an
unbounded loop that ends in memory exhaustion rather than an error message.

The guard above it does not cover this: it rejects a layer that is missing the
placeholder, and says nothing about the content being substituted in.

Suggested fix — scan left to right and never re-scan what was already substituted, which
also preserves the multi-placeholder behaviour the loop exists for:

out=""; rest="$layer_content"
while [[ "$rest" == *'{CORE_TEMPLATE}'* ]]; do
    out="${out}${rest%%\{CORE_TEMPLATE\}*}${content}"
    rest="${rest#*\{CORE_TEMPLATE\}}"
done
layer_content="${out}${rest}"

Reachability / why we are reporting rather than patching. Found while adopting a
Spec Kit-based plugin in a downstream repo. It is not reachable through that plugin: it
ships nothing that declares {CORE_TEMPLATE}, so $content never carries the placeholder
on that path (grep -rl CORE_TEMPLATE over the plugin returns nothing). It is reachable for
any consumer that authors a wrap template layer whose core content includes the literal
token — which is a normal thing to do by accident when a template documents its own
placeholder syntax.