Wrapper GenServer for estimator callback modules.

Responsibilities:

• Resolves parameter references in opts at startup and on parameter changes (mirroring BB.Sensor.Server / BB.Controller.Server).
• Subscribes to the estimator's declared input paths.
• Rejects any envelope stamped with a node other than the local one before it is compared against another envelope's timestamp, since monotonic clocks are node-local. A rejected envelope is dropped with reason :cross_node and is not retained, so a multi-input alias whose newest envelope came from another node stays missing until a local one arrives.
• Dispatches incoming input messages to the callback module via BB.Estimator.handle_input/2. Single-input estimators receive the bare %BB.Message{}; multi-input estimators receive a %{input_name => %BB.Message{}} map gathered when the driver input arrives.
• Enforces max_input_age: an envelope older than its input's budget is discarded before handle_input/2 is called, and the estimator transitions to :degraded with reason :stale_input. Retained non-driver envelopes are re-checked when the driver arrives, since they can age past their budget while they wait. This is independent of latency_budget, which measures callback execution time.
• For multi-input estimators, enforces sync_tolerance: if any non-driver input is stale relative to the driver by more than the configured tolerance, the dispatch is dropped (with [:bb, :estimator, :dropped] telemetry) instead of fired with a stale snapshot.
• Publishes each {output_name, message} returned from a callback's {:reply, outputs, state} reply to that output's configured path.
• Runs the :healthy / :degraded / :lost health state machine and dispatches the configured on_degraded / on_lost / on_recovered commands on each transition. For multi-input estimators only driver arrivals reset the lost_after timer, so a live auxiliary stream cannot mask a driver that has stopped publishing.
• Emits :input, :output, :latency, :dropped, and :transition telemetry. Every timing measurement is in nanoseconds, the same unit as %BB.Message{}.monotonic_time.

Init args

The framework supplies the following keys in the start-link init arg. Internal keys (double-underscored) are stripped by the server before calling BB.Estimator.init/1; public keys (:bb, :estimator_context) are passed through unchanged so user code can read them.

• :__callback_module__ - the user's estimator module.

• :__estimator_inputs__ - `%{mode: :single	:multi, inputs: [...],
  sync_tolerance_ns: integer()	nil}` describing the input wiring. Each

  input spec carries :name, :path, :driver? and a resolved :max_input_age_ns.
• :__estimator_outputs__ - %{output_name => [atom()]} mapping output names to their full pubsub paths.
• :bb - %{robot: module, path: [atom]}, the per-process context.
• :estimator_context - the BB.Estimator.Context.t() exposed to the callback module's init/1.

Plus any user options declared via the estimator's options_schema/0.