Match duplicate candidates literally, not as a regex

Artist and title strings are full of regex metacharacters, so a new candidate could match
an unrelated history entry and be silently skipped as already seen.

Kernel/decision_engine.sh

@@ -15,6 +15,20 @@
 # Currently paired with: Arrs_Stack/playback_aware_lidarr_discovery.sh
 #
 # ==============================================================================================
+# OPERATIONAL MODEL
+# ==============================================================================================
+#
+# A library, not a program. Consumers source it and drive the pipeline themselves:
+#
+#   1. score_candidate()      — sum the four weighted component scores
+#   2. apply_temporal_decay() — reduce by one unit per 30 days of age, floored at 0
+#   3. is_duplicate_candidate() — check the consumer's own history file
+#   4. make_decision()        — ACCEPT or REJECT against the consumer's threshold
+#
+# Every input is supplied by the caller and every output is returned to it. The engine holds
+# no state between calls, reads no config, and never acts on its own verdict.
+#
+# ==============================================================================================
 # DESIGN PRINCIPLES
 # ==============================================================================================
 #
@@ -38,6 +52,60 @@
 #   the consumer's concern — the engine never acts on its own verdict.
 #
 # ==============================================================================================
+# OPERATIONAL SAFEGUARDS
+# ==============================================================================================
+#
+# No Side Effects — Structural, Not Incidental
+#   Writes no files, makes no API calls, deletes nothing, starts nothing. This is the
+#   safeguard: a scoring mistake here can only ever produce a wrong number, never a wrong
+#   action. Keep it that way — the moment this library acquires a side effect, every consumer
+#   inherits it silently.
+#
+# No Root, No Lock, No detect_hosts — Deliberate
+#   Correct for a sourced library and should not be "fixed" to match the executable scripts.
+#   There is no state for a lock to protect, no privileged operation to gate, and no
+#   host-specific config to alias. It runs entirely inside the caller's process.
+#
+# Caller Owns the Verdict
+#   ACCEPT/REJECT is a return value, not an instruction. Nothing here can cause a candidate
+#   to be added, removed, or downloaded — the consumer decides what a verdict means.
+#
+# Literal Duplicate Matching
+#   is_duplicate_candidate() matches with grep -Fx, treating the candidate as data rather
+#   than a pattern. Artist and title strings routinely contain regex metacharacters, and a
+#   false positive here silently discards a genuinely new candidate.
+#
+# Decay Floors at Zero
+#   apply_temporal_decay() clamps at 0, so an old signal can never become a negative score
+#   that drags an otherwise-passing candidate below threshold.
+#
+# Integer Arithmetic Throughout
+#   All scoring is integer. No floating point means no locale-dependent decimal parsing and
+#   no rounding drift between hosts.
+#
+# ==============================================================================================
+# CONFIGURATION
+# ==============================================================================================
+#
+# None, by design. The engine reads no conf file and no environment variable.
+#
+# Thresholds, weights and strictness profiles live with the consumer — see
+# LIDARR_DISCOVERY_* / SONARR_DISCOVERY_* / RADARR_DISCOVERY_* in master.conf. That is what
+# keeps the core domain-agnostic: music strictness cannot leak into TV intake, because the
+# engine never learns which domain it is scoring for.
+#
+# ==============================================================================================
+# RUNTIME MODES
+# ==============================================================================================
+#
+# None — this file is sourced, never executed:
+#
+#   source "$SCRIPT_DIR/../Kernel/decision_engine.sh"
+#
+# It has no argument parsing, no --dry-run and no --status, because it takes no action that
+# a dry run could suppress.
+#
+# ==============================================================================================
 # FUNCTIONS
 # ==============================================================================================
 #
@@ -111,7 +179,12 @@ is_duplicate_candidate() {
     local candidate="$1"
     local history_file="$2"
 
-    grep -qi "^${candidate}$" "$history_file" 2>/dev/null
+    # -F -x, not "^$" anchors: the candidate is data, not a pattern. Interpolating it into a
+    # regex makes every metacharacter in an artist or title active — "R.E.M." matches "RxExMy",
+    # and an unbalanced bracket makes grep error out entirely. Either way the caller reads the
+    # result as "already seen" and silently skips something genuinely new. -F disables regex,
+    # -x anchors the whole line, which is exactly what the anchors were reaching for.
+    grep -qiFx -- "$candidate" "$history_file" 2>/dev/null
 }
 
 # ── make_decision ─────────────────────────────────────────────────────────────