Add named Marks and cross-room NPC pathfinding

- Mark: a debug non-collidable named destination (pixel-art target icon),
  editor tool + name modal, globally-unique names, programmatic Room:addMark
- NPCs walk to Marks: each RGB copy is a per-room resident (like the player),
  running its own A* on its own colour layer, transferring rooms as it crosses
  seams; rendered and collided by the room it currently stands in
- Player copies are obstacles in NPC pathfinding
- A* is footprint-aware for multi-cell NPCs
- Editor: click an NPC to open a filterable destination picker
- Ink: Dialogue.open binds walk_to; Jorge can be sent to "end"/"begin"
- Fix: a blocked NPC now replans each tick and resumes instead of freezing
- [npc] move diagnostics under DEBUG (start/goal/route or block reason)
- Docs (adding-npcs) + CLAUDE.md LÖVE 11.x/float-colour corrections
- Fix test harness loadSprite stub; movement tests green (28)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/adding-npcs.md

@@ -38,11 +38,14 @@ In `main.lua`, next to `startSignBook`:
 
 ## 4. Make the entity
 
-In `npc.lua`, copy `Sign` — pick your art and name your book:
+In `npc.lua`, copy `Sign` — pick your art and name your book. Include `Walker`
+too (it's what lets the NPC path to a Mark; see "Making an NPC walk" below), and
+call `self:initWalker()` after `Entity.initialize`:
 
 ```lua
 Oracle = class("Oracle", Entity)
 Oracle:include(Dialogable)
+Oracle:include(Walker)
 
 function Oracle:initialize(x, y)
 	self.book = oracleBook            -- from step 3
@@ -55,6 +58,7 @@ function Oracle:initialize(x, y)
 			blue  = loadSprite("art/npc/oracle_b.png"),
 		},
 	})
+	self:initWalker()
 end
 ```
 
@@ -82,3 +86,44 @@ Then add it to a room's `rooms/<room>.sav` `objects` list:
 That's it — walk into it and it talks. NPCs are stamped into every colour's
 collision matrix, so they block movement and obey the world's colour rules (a
 red-only player reads them in red).
+
+# Marks — named destinations
+
+A **Mark** is a debug non-collidable: an editor-only annotation the game reads
+back. It carries a globally-unique name, and it's the only thing an NPC can be
+sent to. It is *not* a Note — it just shares the debug overlay toggle. In the
+editor, click the **target** tool at the bottom of the panel, then click a cell
+to drop one and type its name (colliding names auto-suffix `_2`, `_3`, …).
+Left-click an existing mark to rename it; right-click to delete. Marks show only
+under `DEBUG` while the overlay is on (`Tab`).
+
+You can also create marks from code so they exist without living in a `.sav`:
+
+```lua
+room:addMark("gate", 5, 9)   -- registers + renders in debug; skipped by serialize
+```
+
+# Making an NPC walk
+
+Any NPC that includes `Walker` (see step 4) can be sent to a Mark. From Ink, call
+the bound external function — e.g. to leave once the conversation ends:
+
+```ink
+Oracle: The way is open now. Go.
+~ walk_to("gate")
+-> END
+```
+
+`walk_to(name)` is wired for every Dialogable (`dialogable.lua`); it resolves the
+Mark by name and calls `npc:sendTo`. Under the hood the NPC becomes the player's
+RGB model — up to three colour copies, each running its **own** A* on its own
+collision layer (`pathfinding.lua`), stepping in real time and pathing across
+room seams over the world grid. Copies split apart and additively re-merge to
+white on arrival. Pathfinding is footprint-aware, so a 2×2 NPC routes its whole
+body around obstacles rather than threading a 1-wide gap.
+
+## Testing without a story
+
+You don't need an Ink book to try it: in the editor (`DEBUG`), **left-click an
+NPC** to open the destination picker — a filterable list of every Mark. Type to
+filter, click a row (or Enter for the top match) to send it; Esc cancels.