docs(wake): #953 dead-letter retention recorded as load-bearing at the write site

The RETENTION IS LOAD-BEARING comment sits directly above _quarantine_entry()
in digest.sh — the actual writer of dead-letter.jsonl (issue #953 names
store.sh, but store.sh only READS the ledger; the write site is here).

The ledger is append-only history AND the sole evidence base for store.sh
quarantine-audit's conviction predicate (#946). Rotation/pruning/caps would
silently convert provable rows into unprovable ones with no signal at either
end — the audit's clean sweep reads identically before and after evidence
disappears. The comment records the requirements any future retention limit
must ship with (loud prune-time signal + audit residual-class reporting
updated in the same change), and that nothing pruning this file is a
recorded decision, not an omission.

Comment-only: zero behavior delta. manifest.txt deliberately NOT touched —
keeps the declared #964/#967 version collision two-way.

Closes #953

Written-by: pepper (sb-it-1-dt)
Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NsKce8iZuSuRnu3gVMCBKB

packages/mosaic/framework/tools/wake/digest.sh

@@ -488,6 +488,19 @@ _dlq_alarm_offhost() {
 # A dead-letter write failure is itself alarmed (both legs) but never aborts the
 # drain — the good entries MUST still deliver (availability is the whole point of
 # #920).
+#
+# RETENTION IS LOAD-BEARING (#953) — read BEFORE adding rotation/pruning/caps:
+# this ledger is append-only history AND the SOLE evidence base for
+# store.sh quarantine-audit's conviction predicate (#946): a false
+# consumed-hashes witness row is provable ONLY while its matching entry
+# survives HERE. Any rotation, pruning, or size cap — however locally correct
+# — silently converts provable rows into unprovable ones, and the audit's
+# clean sweep reads IDENTICALLY before and after the evidence disappears (no
+# signal at either end; the audit never guesses, by design). If retention
+# limits ever become genuinely necessary, they MUST ship with (a) a loud
+# signal at prune time naming what evidence is being given up, and (b) the
+# audit's residual-class reporting updated IN THE SAME CHANGE. Until then:
+# nothing prunes this file, and that is a recorded decision, not an omission.
 _quarantine_entry() {
   local line="$1" seq="$2" dlq="$STATE_DIR/dead-letter.jsonl"
   if [ ! -f "$dlq" ] || ! grep -qxF "$line" "$dlq" 2>/dev/null; then