Stage 4: upgrades, and a quartermaster to spend them at

Every level banks one choice. Choices queue, and are spent at an NPC in the
hub: walk to it, press E, take one of three weighted options. Seven upgrades,
all data — split shot, glass cannon, spread, sniper, doubleshot, poison,
eraser — and SimWorld gained no per-upgrade branch to run any of them.

The four ambiguities in the brief were settled with the user first, since
each changes what gets written:

  damage      base x (1 + sum additive) x product multiplicative. The flat
              +5% every upgrade carries, spread's -10%, doubleshot's -50%
              and glass cannon's +100% pool; sniper multiplies on top, so
              two snipers is 4x and not +200%.
  glass       half the LEVELLED maximum, multiplying if taken twice, so the
              price does not fade to a rounding error by level 15.
  poison      independent stacks, not a refresh.
  split       +/-45 degrees from the original heading.

Independent poison stacks sound expensive and are not: every dose lasts the
same number of ticks, so doses expire in the order they were added, the
pending expiries are a plain FIFO, and PoisonTrack only ever looks at its
front. O(1) per actor per tick however many are live.

Stats are derived from the upgrade list and never stored, the way level is
derived from experience -- a saved stat cannot disagree with the upgrades
that produced it. Upgrade riders (split charges, poison, erase chance) travel
on the bullet instead, because a shot in flight has to keep what it was fired
with rather than gaining Poison because the shooter just took it.

Two invariants this collided with, both now pinned:

  - bullet speed gained a ceiling. Wall collision samples once per tick, so
    anything over a tile per tick tunnels; two snipers asked for 2480 u/s
    against a 1920 threshold, and a tunnelling bullet looks like a bullet.
  - BULLET_INTEREST_RADIUS rose to 2900, because an upgraded player shot is
    now the longest-travelling bullet in the game. test_interest measured
    the worst case from static content, which upgrades quietly invalidated.

Choosing is intent checked three ways: a choice must be owed, the index must
name one of the three options the SERVER put on the table, and the player
must be standing at the NPC. The offer is rolled once and persisted, so
closing the screen is not a reroll and neither is a crash.

tools/diag_upgrades.tscn covers level -> banked choice -> refused in a
dungeon and refused across the room -> taken at the NPC -> new stats ->
on disk. Bots never walk to the quartermaster, so the smoke test cannot.

Known gap recorded in the roadmap: at PLAYER_BULLET_DAMAGE = 6, the +5% the
first upgrade carries rounds back to 6 and visibly does nothing. It comes out
right in aggregate, but the fix is a balance edit across content.gd and so is
the user's call.

check.sh clean, 357 tests, SMOKE PASS (18 assertions), all four diagnostics
green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/net/net_codec.gd

@@ -462,6 +462,55 @@ static func decode_portals(data: PackedByteArray) -> Array[Dictionary]:
 	return out
 
 
+# --- Upgrade state ----------------------------------------------------------
+# Sent to one peer when its played character's upgrades change: a level gained,
+# a choice spent, a character swapped in. Cold, and about the played character
+# only -- nobody needs to know what anyone else has taken.
+
+static func encode_upgrade_state(pending: int, offer: Array[StringName],
+		taken: Array[StringName]) -> PackedByteArray:
+	var b := StreamPeerBuffer.new()
+	b.big_endian = false
+	b.put_u8(clampi(pending, 0, 255))
+	b.put_u8(mini(offer.size(), 255))
+	for id in offer:
+		b.put_u8(Upgrades.index_of(id))
+	b.put_u16(mini(taken.size(), 65535))
+	for id in taken:
+		b.put_u8(Upgrades.index_of(id))
+	return b.data_array
+
+
+## Returns { "pending": int, "offer": Array[StringName], "taken": Array[StringName] }.
+static func decode_upgrade_state(data: PackedByteArray) -> Dictionary:
+	var offer: Array[StringName] = []
+	var taken: Array[StringName] = []
+	var out := {"pending": 0, "offer": offer, "taken": taken}
+	if data.size() < 2:
+		return out
+	var b := StreamPeerBuffer.new()
+	b.big_endian = false
+	b.data_array = data
+	out["pending"] = b.get_u8()
+	var offer_count := b.get_u8()
+	for _i in offer_count:
+		if b.get_available_bytes() < 1:
+			return out
+		var id := Upgrades.by_index(b.get_u8())
+		if not id.is_empty():
+			offer.append(id)
+	if b.get_available_bytes() < 2:
+		return out
+	var taken_count := b.get_u16()
+	for _i in taken_count:
+		if b.get_available_bytes() < 1:
+			return out
+		var id := Upgrades.by_index(b.get_u8())
+		if not id.is_empty():
+			taken.append(id)
+	return out
+
+
 # --- Character roster -------------------------------------------------------
 # Sent once at login and after any change. Low frequency and carries strings,
 # like the online roster, so it is the same fixed-header-then-utf8 shape.