Permission-gated step bridge for sandboxed mods

The sandbox blocks love.system and love.filesystem, which orphans the
native step bridge (#452, #489): its one consumer can no longer call
syncHealthSteps or read steps_pending.json (#1186).

Adds a "steps" manifest permission (shown to the player like the
others) gating a mod.steps facade: available() probes the bridge
quietly, sync() forwards the async refresh, poll() hands the mod its
copy of a delivery. The engine owns the pending file -- mods never name
a path and receive only { steps, from, to }. Without the permission the
acting calls name it, following the network gate. No new events, hooks
or registries; nothing removed.

RFC 0009. Tests: tests/modkit/cases/steps_bridge.lua (no-mod cold
bridge, permissioned sync/poll, per-mod copies, contract-field
filtering, malformed-delivery drop, unpermissioned refusal, bridgeless
build).

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

src/mods/Steps.lua

@@ -0,0 +1,84 @@
+-- The scoped seam for the native step bridge (#1186).
+--
+-- The iOS/Android builds count the player's real-world steps natively
+-- (#452, #489) and deliver them by writing steps_pending.json into the
+-- save-directory root.  Before the sandbox, the Pokéwalker mod called
+-- love.system.syncHealthSteps() and consumed that file itself; the sandbox
+-- blocks both, which is correct -- love.system launches URLs and the file
+-- API names paths -- but it left the bridge with no consumer at all.
+--
+-- This module is the narrow replacement, gated by the "steps" permission
+-- in manifest.json (the network model: a permission the player sees that
+-- genuinely gates a capability).  The engine owns the file: mods never
+-- learn its name or location, they receive only the three contract fields
+-- ({ steps, from, to }), each permissioned mod gets its own copy, and the
+-- merge-don't-overwrite anchor semantics stay on the native side where
+-- they always lived.
+--
+-- No frame pump: the file is looked for lazily when a mod polls, so a
+-- build with no permissioned mod installed never touches the bridge or
+-- the disk.
+
+local Json = require("src.link.Json")
+
+local Steps = {}
+
+-- The native contract's drop point, in the save-directory root (see
+-- mobile/ios and mobile/android step bridges).
+Steps.PENDING = "steps_pending.json"
+
+local function bridge()
+  return _G.love and _G.love.system and _G.love.system.syncHealthSteps
+end
+
+-- Whether this build carries the native bridge.  Desktop builds do not;
+-- a mod uses this to stay dormant without probing love.system.
+function Steps.available()
+  return bridge() ~= nil
+end
+
+-- Ask the native side to refresh its count.  Async: the result lands in
+-- the pending file and comes back through a later poll.  The platform's
+-- own consent sheet (HealthKit / ACTIVITY_RECOGNITION) still appears on
+-- first use, exactly as it did pre-sandbox.  false when there is no
+-- bridge to ask.
+function Steps.sync()
+  local fn = bridge()
+  if not fn then return false end
+  fn()
+  return true
+end
+
+-- Consume the pending file, if one has appeared, and fan its payload out
+-- to every permissioned mod's queue.  Only the contract fields travel;
+-- anything else in the file stays in the file's grave.  A malformed or
+-- empty delivery is dropped whole -- the native anchor only advances on a
+-- successful sync, so nothing is lost to a bad write.
+function Steps.pump(loader)
+  local fs = _G.love and _G.love.filesystem
+  if not (fs and fs.getInfo(Steps.PENDING, "file")) then return end
+  local raw = fs.read(Steps.PENDING)
+  fs.remove(Steps.PENDING)
+  if not raw then return end
+  local ok, decoded = pcall(Json.decode, raw)
+  if not ok or type(decoded) ~= "table" then return end
+  local steps = tonumber(decoded.steps)
+  if not steps or steps <= 0 then return end
+  local payload = { steps = steps, from = decoded.from, to = decoded.to }
+  for _, queue in pairs(loader.stepsQueues) do
+    queue[#queue + 1] = { steps = payload.steps, from = payload.from,
+                          to = payload.to }
+  end
+end
+
+-- The next delivery for this mod, or nil.  Each permissioned mod consumes
+-- its own queue, so two mods both see the same walk (pre-sandbox, whoever
+-- read the file first won).
+function Steps.poll(loader, modId)
+  Steps.pump(loader)
+  local queue = loader.stepsQueues[modId]
+  if not queue then return nil end
+  return table.remove(queue, 1)
+end
+
+return Steps