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/Manifest.lua

@@ -10,7 +10,8 @@ local Version = require("src.core.Version")
 local Manifest = {}
 
 Manifest.PROFILES = { content = true, overhaul = true, total_conversion = true }
-Manifest.PERMISSIONS = { network = true, filesystem = true, engine_internals = true }
+Manifest.PERMISSIONS = { network = true, filesystem = true,
+                         engine_internals = true, steps = true }
 
 -- link-relevant registries; a mod that writes into one of these while
 -- declaring affects_link = false gets an attributed warning from the loader