fix(spoolman): repair dead slot-map persistence + isolate it per printer

The AMS-slot -> Spoolman-spool persistence never worked: KobraXBridge
referenced `config_loader` in both the load (__init__) and save
(handle_kx_spoolman_set_active) paths, but the module alias is `env_loader`
(kobrax_moonraker_bridge.py:32). The resulting NameError was swallowed by a
bare `except`, so the map was neither loaded on startup nor written on change
- it only appeared to persist.

The map also lived in a single global `[spoolman] slot_spools` key, so on a
multi-printer bridge two AMS units clobbered each other's mapping (same class
of bug as #74/#75 for filament profiles).

- config_loader: add list_spool_map()/save_spool_map(printer_id) using a
  per-printer `[spoolman_<id>]` section with read-fallback to the legacy
  global key, mirroring _filament_section/list_filament_profiles. The global
  `[spoolman]` section keeps server/sync_rate.
- bridge: load via config_loader.list_spool_map(self._printer_id); persist via
  save_spool_map(..., self._printer_id); surface failures via log.warning
  instead of a silent except.
- _build_mmu_object: emit real gate_spool_id from the per-printer map (was
  hardcoded [-1]*num_gates) so Happy-Hare/OrcaSlicer can show the bound spool.
- config.ini.example: document the [spoolman] section.
- tests: tests/test_spoolman_slot_map.py (per-printer isolation, persistence
  round-trip, server/sync_rate preservation, parser robustness).

Verified on a 2-printer bridge: after restart KX1 loads its spools and KX2
loads its own, isolated; a real multicolor print deducted per slot (white spool
1.02g vs 0.98g slicer estimate) against the correct printer's spools.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

config.ini.example

@@ -41,6 +41,21 @@ web_upload_warning = 1
 # Poll-Intervall in Sekunden
 poll_interval    = 3
 
+# ─── Spoolman (optional) ───────────────────────────────────────────────────────
+# Verfolgt den Filamentverbrauch je AMS-Slot und bucht ihn automatisch vom
+# passenden Spool ab (mm-basiert, wie Moonraker; Spoolman rechnet mm→Gramm).
+# [spoolman]
+# # Server-URL der Spoolman-Instanz (aus Sicht des Bridge-Containers erreichbar):
+# server    = http://192.168.x.x:7912
+# # 0 = nur am Druckende abbuchen, >0 = alle N Sekunden während des Drucks:
+# sync_rate = 0
+#
+# Die AMS-Slot → Spool-Zuordnung wird in der Weboberfläche gesetzt und je Drucker
+# automatisch persistiert (nicht von Hand eintragen):
+#   Einzeldrucker : [spoolman]   slot_spools = 0:42,1:17
+#   Multi-Printer : [spoolman_1] slot_spools = 0:42,1:17
+#                   [spoolman_2] slot_spools = 0:5,1:6
+
 # ─── Multi-Printer (optional) ──────────────────────────────────────────────────
 # Mehrere Drucker können als [printer_1], [printer_2], … definiert werden.
 # Jede Bridge-Instanz verbindet sich mit einem Drucker (je eigener Port).