fix: Type-Aware SCALE-to-JSON Serializer

[eugypalu]

Replaces the two-phase event serialization pipeline (decode → post-process) with the same single-phase, type-aware ScaleVisitor already used by extrinsics.

What changed

decode/args.rs

• Added a second const generic PRESERVE_ENUM_CASE to ScaleVisitor to control basic enum variant casing
• New EventJsonVisitor type alias: ScaleVisitor<true, true> — camelCase keys + preserved enum casing (e.g. "Mandatory", "Yes" instead of "mandatory", "yes")
• Existing JsonVisitor and CallArgsVisitor behavior unchanged (PRESERVE_ENUM_CASE = false)

decode/events.rs

• Removed FieldWithTypeExtractor, ValueExtractor, EventField, is_basic_enum, lowercase_first_char
• Removed all post-processing functions: convert_bytes_to_hex, transform_json_unified, try_convert_accountid_to_ss58
• Threaded ss58_prefix through the visitor chain (EventsVisitor → EventRecordVisitor → PalletEventVisitor → ActualEventVisitor)
• ActualEventVisitor now decodes fields directly with EventJsonVisitor

processing/events.rs

• Removed all post-processing logic (AccountId32/MultiAddress/RewardDestination special-casing, transform_json_unified calls)
• Event fields arrive fully transformed from the decoder — no second pass needed

closes #41