ContentBlockDeltaEvent

Emitted for each incremental update within a content block.

The delta field carries the incremental content block update. Accumulation rules:

• text-delta → append text to the active block's text field
• reasoning-delta → append reasoning to the active block's reasoning field
• data-delta → append data to the active block's data field
• tool-call-delta → append args to the active tool-call block's args field
• block-delta → shallow-merge fields onto the active block

Consumers ignore delta types they don't recognize. content-block-finish carries the complete block.

A tool call's args stream in one of two forms, never both in one block: tool-call-delta slices, which append, or block-delta snapshots of the args so far, which replace.