[Tier 4] Make reconciliation cosine threshold and EMA alpha configurable

[fmasi]

Symptom: No tuning available for cross-chunk speaker reconciliation sensitivity.

Root cause

TranscriberCore/SpeakerReconciler.swift:43 hardcodes the cosine threshold at 0.65 and :102 hardcodes the EMA alpha at 0.9; neither is tunable.

Proposed fix

• Surface both values via Config, using the current hardcoded values (0.65 / 0.9) as defaults.

Acceptance criteria

• Both values configurable with current defaults preserved.
• reconcile signature and tests updated.