Adv 2.0 D&D Phase 12 E2c: Siege Mode effects + threat-70 warning

§8.3 siege economics:
- applyDailyBurn now takes an explicit siege flag and enforces a 2× floor
  on supply burn even when HarshMod is below 2 (tier-1 zones still get
  starved out per spec).
- currentBurn mirrors the same precedence so status/briefing readouts
  stay consistent.
- Briefing rollover passes e.SiegeMode through, decoupling siege from
  the harsh-conditions composite.

§8.3 threat-70 warning: applyDailyThreatDrift emits a one-time
appendApproachingSiegeLog when the level crosses 70 (prevLevel<70 and
new level≥70). Pulls from a new flavor.ThreatClockApproachingSiege pool
seeded with the spec's verbatim warning beat plus two voice-matched
alternates.

Other siege effects (boss +20 HP / Legendary Resistance, cleared-room
respawn, no-short-rest enforcement) stay deferred to the combat-link
phase — ThreatBandInfo already exposes the flags that engine will read.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/dnd_expedition_threat.go

@@ -113,6 +113,7 @@ func applyDailyThreatDrift(e *Expedition) (int, string, error) {
 	if delta == 0 {
 		return 0, reason, nil
 	}
+	prevLevel := e.ThreatLevel
 	prevBand := threatBandFor(e.ThreatLevel, e.SiegeMode)
 	if err := applyThreatDelta(e.ID, delta, reason); err != nil {
 		return 0, reason, err
@@ -132,9 +133,22 @@ func applyDailyThreatDrift(e *Expedition) (int, string, error) {
 	if newBand != prevBand && newBand > prevBand {
 		_ = appendThreatTransitionLog(e, newBand)
 	}
+	// E2c: §8.3 — one-time warning when crossing 70.
+	if prevLevel < 70 && e.ThreatLevel >= 70 {
+		_ = appendApproachingSiegeLog(e)
+	}
 	return delta, reason, nil
 }
 
+// appendApproachingSiegeLog records the §8.3 "begin warning at 70" beat
+// once per expedition. Caller is responsible for only firing it on the
+// crossing edge (see applyDailyThreatDrift).
+func appendApproachingSiegeLog(e *Expedition) error {
+	line := flavor.Pick(flavor.ThreatClockApproachingSiege)
+	return appendExpeditionLog(e.ID, e.CurrentDay, "threat",
+		"threat clock past 70 — siege approaches", line)
+}
+
 // appendThreatTransitionLog records a band-crossing in the expedition log
 // with the prewritten TwinBee narration for that band.
 func appendThreatTransitionLog(e *Expedition, band ThreatBand) error {