feat(vagrant): backpack as inventory container — end-to-end pickup/drop demo

- Add container={kind="list"} to the backpack template so inventory.add
  accepts it as a valid list-container
- Remove old backpack proximity-trigger (print-only callback, pre-B.5);
  the rock's register_pickup is now the sole interaction trigger
- Emit unconditional inventory inspection trace after every add/remove:
  backpack count + item reg-ids for user-facing demo feedback
- Add CI-gated tree-check traces: tree_check_after_pickup=ok and
  tree_check_after_drop=ok verify composition-tree child placement
- Bump version 0.7.3 -> 0.8.0 across init.lua header, manifest.module,
  and README
- README: rewrite Controls section into full demo walkthrough; add Demo
  Walkthrough section; update Demonstrates + Interactions sections;
  add v0.8.0 CHANGELOG entry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

init.lua

@@ -1,9 +1,9 @@
--- sporel-module-vagrant-skeleton v0.7.3
+-- sporel-module-vagrant-skeleton v0.8.0
 -- Sprite-mode test-chamber: 13-bone humanoid puppet + sprite-tilemap +
 -- hardcoded furniture. Subterrain-style puppet control model:
 -- multi-look-target (aim/soft), slerp body rotation, hip-static legs,
 -- foot-body-orientation. Sprint (shift+wasd) + interaction (E near
--- backpack) added v0.4.4.
+-- pickup-item) added v0.4.4.
 --
 -- v0.6.0 (Phase A.4): Backpack promoted to composition-Entity (Atlas-UV
 -- render-path via render.draw_entities{tag="renderable"}). Bed + Bench
@@ -15,6 +15,13 @@
 -- via puppet.move_to. Movement-Logic moves the actor; puppet renders.
 -- Untagged actor — render.draw_entities doesn't see it; puppet handles
 -- the render via puppet.render(state.player) unchanged.
+--
+-- v0.8.0: Backpack template gains container={kind="list"}.
+-- End-to-end pickup/drop loop: E on rock → inventory.add(backpack, rock)
+-- → rock disappears from draw_entities; Q → rock reappears at player
+-- position. engine.print shows inventory count + item-IDs after each
+-- add/remove. Tree-check (CI-gated) verifies child placement in
+-- composition tree.
 
 local input       = require("lib-core.input")
 local camera      = require("lib-core.camera")
@@ -37,11 +44,13 @@ local ANIM_SPEED_SPRINT = 0.9        -- 2x
 -- Furniture positions (Bed/Bench remain hardcoded for v0.1; Backpack
 -- promoted to Inactive-Entity via composition. Bed/Bench have no
 -- interaction-trigger and no demo-pflicht to be entities — they stay
--- atlas-furniture until Phase B (container-on-bed) or similar triggers).
+-- atlas-furniture until a container-on-bed feature or similar triggers).
 local BACKPACK_X, BACKPACK_Y = 350, 400
 local INTERACTION_RANGE = 40         -- pixels
 
 -- Backpack-Template (Phase A.4 — Atlas-UV render-path).
+-- v0.8.0: container={kind="list"} declared so the backpack receives
+-- items via inventory.add (capability-by-declaration).
 -- Real sprite_atlas + sprite_uv values are filled in M.init after the
 -- atlas-JSON is parsed; defaults here are placeholders so the inert-
 -- declaration types are correct (string + numbers).
@@ -53,6 +62,7 @@ composition.define_template{
         position     = {x = 0, y = 0},
     },
     tags = {"renderable"},
+    container = { kind = "list" },   -- receives items via inventory.add
 }
 
 -- Rock-Template: an item sitting in the world that the player can pick up.
@@ -85,7 +95,7 @@ composition.define_template{
     tags = {},                         -- intentionally untagged: not renderable
 }
 
--- Item-Template-Convention (Phase B):
+-- Item-Template-Convention:
 -- An "item" is a composition-template with:
 --   - properties.stack_mode = "individual"     (required by inventory-list)
 --   - properties.sprite_atlas + sprite_uv      (Atlas-UV render path)
@@ -101,8 +111,8 @@ composition.define_template{
 -- re-parents it to the world at the player position, and re-registers a
 -- pickup-trigger at the new location.
 --
--- Rock template (B.4) and container block on backpack (B.5) activate this
--- convention end-to-end. B.3 establishes the glue only.
+-- Rock template (v0.7.3) and container block on backpack (v0.8.0) activate this
+-- convention end-to-end. The pickup/drop glue (v0.7.2) sets up the wiring.
 
 local state = {
     player = nil,                -- puppet handle (state-bearing for animation)
@@ -138,7 +148,7 @@ local WALK_ANIMS = {"walk_fwd_lower", "walk_back_lower", "walk_left_lower", "wal
 -- When the player presses E within INTERACTION_RANGE, the item is moved
 -- into state.backpack via inventory.add (renderable-tag flips off) and
 -- the trigger is unregistered so the item no longer shows as interactable.
--- Called by B.4 after placing the rock, and by drop-glue after relocation.
+-- Called after placing the rock, and by drop-glue after relocation.
 local function register_pickup(item)
     local x = item:get_property("position.x")
     local y = item:get_property("position.y")
@@ -148,6 +158,27 @@ local function register_pickup(item)
             inventory.add(state.backpack, item)
             interaction.unregister(trigger_id)
             if CI_FRAME_PERF then engine.print("vagrant: event=pickup") end
+
+            -- Inventory inspection trace (unconditional — user-facing demo output).
+            local count = inventory.count(state.backpack)
+            local items = inventory.contents(state.backpack)
+            local ids = {}
+            for _, it in ipairs(items) do
+                table.insert(ids, tostring(it:get_property("composition.reg_id")))
+            end
+            engine.print(string.format("vagrant: backpack count=%d items=[%s]",
+                count, table.concat(ids, ",")))
+
+            -- Tree-check (CI-gated): verify the item was actually placed as a
+            -- child of the backpack entity in the composition tree.
+            if CI_FRAME_PERF then
+                local found = false
+                for slot_name, _ in pairs(state.backpack:get_children()) do
+                    if slot_name:match("^item%.%d+$") then found = true; break end
+                end
+                engine.print(string.format("vagrant: tree_check_after_pickup=%s",
+                    found and "ok" or "MISSING"))
+            end
         end)
 end
 
@@ -368,18 +399,9 @@ function M.init(ctx)
     input.bind("drop",       { "q" })       -- drop most-recent item from backpack
     input.bind("quit_to_launcher", { "escape" })
 
-    -- Proximity trigger on the backpack-Entity. Anchor read from the
-    -- entity's position-Properties; sign is stationary so a snapshot is
-    -- correct (interaction-API takes raw numbers, not closures, in v0.1).
-    interaction.register(
-        state.backpack:get_property("position.x"),
-        state.backpack:get_property("position.y"),
-        INTERACTION_RANGE, "interact",
-        function(ctx)
-            engine.print(string.format(
-                "vagrant: interact with backpack (distance=%.1f)",
-                ctx.distance))
-        end)
+    -- Note: no backpack proximity-trigger registered here. The backpack is now
+    -- a container; interaction is driven by the rock's register_pickup
+    -- trigger. The old backpack-print callback has been removed.
 
     -- CI trace: assets loaded count (4 atlas textures: player + tiles + world + stone).
     if CI_FRAME_PERF and not ci_assets_traced then
@@ -411,6 +433,27 @@ function M.update(ctx, dt)
             item:set_property("position.y", py)
             register_pickup(item)
             if CI_FRAME_PERF then engine.print("vagrant: event=drop") end
+
+            -- Inventory inspection trace (unconditional — user-facing demo output).
+            local count = inventory.count(state.backpack)
+            local items = inventory.contents(state.backpack)
+            local ids = {}
+            for _, it in ipairs(items) do
+                table.insert(ids, tostring(it:get_property("composition.reg_id")))
+            end
+            engine.print(string.format("vagrant: backpack count=%d items=[%s]",
+                count, table.concat(ids, ",")))
+
+            -- Tree-check (CI-gated): verify the backpack has no item.* children
+            -- after drop (item was re-parented to world).
+            if CI_FRAME_PERF then
+                local found = false
+                for slot_name, _ in pairs(state.backpack:get_children()) do
+                    if slot_name:match("^item%.%d+$") then found = true; break end
+                end
+                engine.print(string.format("vagrant: tree_check_after_drop=%s",
+                    found and "MISSING" or "ok"))
+            end
         end
     end