Wire client chunk streaming + dedicated-server content load to the backend

Implements the client-side half of docs/architecture/world_persistence_impl.md
(the backend API in backend/app/routers/{world_content,internal}.py and
game_servers.py's --api-url/--server-token/_await_ready were already merged):

- scripts/http_chunk_source.gd: new HttpChunkSource adapter satisfying
  ChunkStreamManager.set_server_rpc()'s seam. Fetches chunks over HTTP from
  the public world_content API (JWT via ApiClient), with ETag/If-None-Match
  (304) support backed by a user://chunk_cache/<checksum> cache, and a
  bounds-query list_chunks() helper (§8.1's chunk index). A 404 (unwritten
  coordinate) renders empty instead of erroring, per §8.1.

- scripts/server.gd: startup now parses --api-url/--server-token, and
  before binding the ENet port fetches the world's manifest + the
  spawn-neighbourhood chunks from /api/internal (§7.2/§7.3/§8.3), decodes
  them, and builds real StaticBody3D collision from solid tiles + non-
  walkable objects. Falls back to test_level.tscn if the backend is
  unreachable, the world has no rooms, or the room has zero chunks -- never
  hangs or crashes. Writes "ready"/"content" (plus diagnostic
  chunks_loaded/collision_shapes counts) into the status file that
  game_servers.py's _await_ready() already reads.

- scripts/api_client.gd: exposes a `base_url` getter so other client code
  building its own requests (HttpChunkSource; server.gd doesn't use this
  autoload at all, since its auth model is the per-spawn server token, not
  a player JWT) can target the same backend.

Also fixes three real, previously-dormant bugs this work's first real
exercise of these code paths uncovered (nothing before this called
encode_binary() or fed real data through set_server_rpc()):

- chunk_data_decoder.gd: `_build_metadata_section` never wrapped its
  output with the section id/length header every other section builder
  uses, so `encode_binary()` produced corrupt payloads the decoder read as
  a single garbage-length section ("truncated section body").
- chunk_stream_manager.gd: `_parse_coord_from_key` assigned a
  PackedStringArray (String.split()'s real return type) to an
  Array[String]-typed variable -- a hard crash.
- chunk_stream_manager.gd: `_instantiate_chunk_scene_with_data` read a
  nonexistent `data.format_version` field; ChunkPayload's field is
  `version`.

Adds scripts/smoke_test_http_chunk_source.gd, a real end-to-end smoke test
(register, create world, PUT a real chunk, stream it back through
ChunkStreamManager + HttpChunkSource, plus the unwritten-coordinate and
304-refetch paths).

Verified against a real backend + Postgres:
- Seeded real chunk content, joined through the real
  GameServerSupervisor spawn path: status file reports
  content=loaded, chunks_loaded=1, collision_shapes=33.
- A room with zero chunks and an unreachable backend (closed port) both
  fall back to test_level.tscn cleanly (content=fallback, ready=true, no
  hang/crash).
- Full existing Godot smoke suite (16 suites, including
  smoke_test_enter_world.gd and smoke_test_network_client.gd) passes with
  no regressions.
- Backend pytest (132/132) untouched and still green -- no backend/
  changes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2FESCaMXeJpC9gSctiEeD

scripts/chunk_data_decoder.gd

@@ -498,7 +498,17 @@ static func _build_metadata_section(payload: ChunkPayload) -> PackedByteArray:
 	data.append_array(_write_float32(payload.bounds.size.x))
 	data.append_array(_write_float32(payload.bounds.size.y))
 	data.append_array(_write_float32(payload.bounds.size.z))
-	return data
+	# Every other section builder wraps its body with `_wrap_section`
+	# (section_id + uint32 length prefix) -- this one didn't, so
+	# `encode_binary()` spliced the raw metadata bytes directly into the
+	# section stream with no header at all. The decoder then read the
+	# metadata's own first bytes (a string length prefix) as a bogus
+	# section_id/section_len pair, producing a garbage length that
+	# overran the buffer ("truncated section body") and silently found
+	# zero real sections. Never caught before because nothing in this
+	# codebase called encode_binary() until
+	# scripts/smoke_test_http_chunk_source.gd.
+	return _wrap_section(data, SECTION_META)
 
 
 static func _build_tile_section(layer: TileLayerData) -> PackedByteArray: