N4/E1: second pet slot for the Tier-4 Estate

An Established (Tier-4) home can draw a second companion. Both pets show up on
the sheet and the town showcase, level via babysitting, and wear their own
barding (!thom pet2buy <tier>).

Combat: both pets contribute their attack/deflect/whiff procs at half weight —
DerivePlayerStats now averages over the active pets, so a pair reads as roughly
one full pet (flavor-forward, not a stat spike; difficulty-neutral). The average
reduces to identity over a single pet (x/1==x, 0.0+x==x, 3+(2L+1)/2==3+L), and
the engines still roll one proc per channel per round, so the single-pet path
and TestCombatCharacterization golden stay byte-identical. Pinned by
TestDerivePlayerStats_OnePetIsByteIdentical + the golden.

Scope kept deliberately flavor-forward: pet 2 carries identity/level/barding and
the three combat procs only. The morning-defense buff, death/ditch-recovery
save, and the level-10 supply-shop unlock stay pet-1 mechanics — no second
defensive multiplier stacks, and the one-pet path is untouched by construction.

Storage: parallel pet2_* columns on player_meta + Pet2* mirror on
AdventureCharacter (absent == no second pet, DEFAULT '' correct for every
pre-existing row, no backfill — the N2-temper / P4-party precedent). Pet-1 code
paths are untouched. Arrival reuses the chase/feed/type/name DM flow, slot-aware,
gated HouseTier>=4 with an established first pet.

Review fixes folded in (high-effort /code-review, 3 angles): pet-2 barding block
no longer hidden when pet-1 is chased away; showcase tie-break restored
(level desc, then name); petGrantXP collapsed onto the shared level-up helper;
dead-param armor-buy wrappers inlined; pet-2 barding tagged with its own euro
ledger reason.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/dnd_sheet.go

@@ -212,6 +212,9 @@ func renderDnDSheet(c *DnDCharacter, adv *AdventureCharacter, meta *PlayerMeta,
 		if adv.PetName != "" {
 			b.WriteString(fmt.Sprintf("  Pet: %s the %s (lv %d)\n", adv.PetName, adv.PetType, adv.PetLevel))
 		}
+		if adv.Pet2Name != "" {
+			b.WriteString(fmt.Sprintf("  Pet: %s the %s (lv %d)\n", adv.Pet2Name, adv.Pet2Type, adv.Pet2Level))
+		}
 		if house.Tier > 0 {
 			b.WriteString(fmt.Sprintf("  Housing: tier %d\n", house.Tier))
 		}