feat: agent map data + framebuffer canvas screenshot

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First two of the three agreed agent-tooling building blocks (data first):

GetMapData(mapId) / MCP get_map_data — compact, agent-oriented JSON projection of a loaded map: a walkability grid (. walkable / # solid / void; folds every visible layer's per-tile solid flags + blocking placements), placements with names resolved through the canonical resolvePlacementDefinition, spawn points and teleports with their targets. Pure engine service (services/agent-map-data.ts, spec'd + registered), works with no open scene, engine or visible window. It proved itself during its own validation: the grid immediately exposed two real gameplay bugs — the tree-house spawn point sat on the solid stool tile, and both kokiri door triggers sat on solid tiles the player can never enter (fixed in this PR: spawn → (5,7), triggers → the walkable mat row 67, return arrival → (76,68)).

Framebuffer canvas screenshot — Screenshot('canvas') now reads the WebGL framebuffer instead of Gtk.WidgetPaintable: the engine widget hooks connect_after('render') on the GLArea bridge (the only moment its framebuffer is bound — out-of-frame readPixels returns blanks, which the first two implementation attempts demonstrated), forces a frame via queue_render(), flips the rows and encodes via GdkPixbuf. Chrome-free content that works while the window is occluded (mapped windows keep their frame clock); widget-snapshot stays as fallback (no engine / minimised). Screenshot is now async on the Control plane (precedent: StartSession). Known quirk noted in TODO: a faint offset ghost from FBO double-buffer residue.

Bridge: new get_map_data tool (+ guidance to prefer it over screenshots), updated screenshot description, pause-policy classification (GetMapData: read-only, pinned table extended). Both validated END-TO-END through the restarted MCP bridge against the live editor. Follow-ups in TODO.md: render_map (offscreen full-map PNG via the MapPreview Gsk pipeline) and an optional blocking open_scene.