fix(teleport): preserve dungeon cell identity on respawn (#215)

Classify teleport landblock transitions from the source and destination Position cell IDs instead of render-space coordinates. This keeps same-dungeon death respawns on their resident floor physics while preserving the existing true cross-landblock streaming path.

Co-Authored-By: Codex <noreply@openai.com>

docs/research/2026-07-13-same-dungeon-respawn-landblock-identity-pseudocode.md

@@ -0,0 +1,115 @@
+# Same-dungeon respawn landblock identity — retail pseudocode
+
+## Symptom and captured cause
+
+After `/die` in the starter dungeon, ACE respawned the player in the same cell,
+`0x8C0401AD`. The player stood correctly until the first movement input, then
+lost floor contact and fell through the world.
+
+The destination update and the player controller both named cell
+`0x8C0401AD`, but acdream reconstructed the source landblock from render-space
+XYZ. The dungeon cell's valid local Y was about `-30.4 m`, so this expression:
+
+```text
+sourceLbY = streamingCenterY + floor(renderY / 192 m)
+          = 4 + floor(-30.4 / 192)
+          = 3
+```
+
+misidentified the source as landblock `(140,3)`. The destination was `(140,4)`,
+so the same-cell respawn entered the cross-landblock branch, removed
+`0x8C04FFFF` from the physics engine, and left the player with no hydrated floor.
+
+Dungeon EnvCells may have arbitrary negative frame origins. Their XYZ values
+therefore cannot identify an AC landblock.
+
+## Named retail oracle
+
+Retail stores cell identity inside every position:
+
+```c
+// acclient.h:30658-30662
+struct Position : PackObj
+{
+    unsigned int objcell_id;
+    Frame frame;
+};
+```
+
+The local-player teleport path preserves that complete value end to end:
+
+- `SmartBox::TeleportPlayer @ 0x00453910` passes its `Position const*` directly
+  to `CPhysicsObj::SetPositionSimple`.
+- `CPhysicsObj::SetPositionSimple @ 0x005162B0` copies that complete `Position`
+  into a `SetPositionStruct`, then calls `CPhysicsObj::SetPosition`.
+- `SmartBox::HandleReceivedPosition`'s local-player teleport branch
+  (`0x0045415F`–`0x00454168`) passes the received `Position` to
+  `SmartBox::TeleportPlayer`; it does not derive a cell from XYZ.
+
+References:
+
+- `docs/research/named-retail/acclient.h:30658`
+- `docs/research/named-retail/acclient_2013_pseudo_c.txt:92514`
+- `docs/research/named-retail/acclient_2013_pseudo_c.txt:93013`
+- `docs/research/named-retail/acclient_2013_pseudo_c.txt:284276`
+
+## Reference-client/server cross-check
+
+The existing L.3 research traces the same contract through two independent
+implementations:
+
+- ACE sends death respawn as `GameMessagePlayerTeleport`, immediately followed
+  by an `UpdatePosition` whose `PositionPack` contains the destination cell and
+  frame (`docs/research/2026-05-04-l3-port/12-hard-teleport-branch.md`,
+  sections 7 and 9).
+- The retail packet path unpacks `PositionPack.position` as
+  `objcell_id + Frame` and routes that complete value into the hard-teleport
+  branch (`docs/research/2026-05-04-l3-port/03-up-routing.md`, sections 0 and 3).
+
+The repository's old external ACE checkout is no longer present after the
+WorldBuilder extraction, so the checked-in cross-reference research is the
+stable ACE source citation for this change.
+
+## Faithful pseudocode
+
+Retail has synchronous cell access and can install the received `Position`
+directly. acdream keeps its asynchronous loading cover, but the identity
+decision must use the same `Position.objcell_id` values:
+
+```text
+on local player teleport destination(receivedPosition):
+    sourceCell = player.currentPosition.objcell_id
+    destinationCell = receivedPosition.objcell_id
+
+    if sourceCell is zero:
+        # acdream-only pre-placement fallback; no XYZ inference
+        sourceLandblock = currentStreamingCenterLandblock
+    else:
+        sourceLandblock = landblockOf(sourceCell)
+
+    destinationLandblock = landblockOf(destinationCell)
+    crossesLandblock = sourceLandblock != destinationLandblock
+
+    if crossesLandblock:
+        evict the old streaming-origin physics landblock
+        recenter streaming on destinationLandblock
+        start the existing destination hydration path
+
+    retain receivedPosition.objcell_id as the pending destination cell
+    when the existing readiness gate opens:
+        place the player with the complete destination cell + frame
+```
+
+`landblockOf(cell)` means `(cell & 0xFFFF0000) | 0xFFFF`. At no point does
+the decision divide or floor position coordinates.
+
+## Conformance cases
+
+1. Source and destination are both `0x8C0401AD`, while render Y is negative:
+   same landblock; do not evict or recenter.
+2. Source and destination are different EnvCells in `0x8C04`:
+   same landblock; do not evict or recenter.
+3. Source `0x8C0401AD`, destination `0xA9B40022`:
+   cross-landblock; retain the existing recenter/hydration behavior.
+4. Source cell is zero before first placement:
+   compare against the current streaming-center ID; never infer from XYZ.