Feature: Support inserting LaTeX equation blocks or media at specific document positions

Problem

When writing academic documentation with mathematical formulas, users need to insert equation blocks at precise positions in the document (e.g., right after explanatory text). Currently docs +media-insert only appends to the document end.

Current Limitations

1. docs +media-insert does not support --index parameter for positional insertion
2. docs +update --mode insert_after works only for text, not media blocks
3. No API support for native Feishu equation blocks (block_type for LaTeX equations)

Requested Features

1. Support --index in docs +media-insert to insert at specific positions
2. Support inserting equation/LaTeX blocks at specific positions
3. Support insert_after mode with media files

Example Use Case

Writing paper research notes requiring formulas like:

• Token Merger: h_{v_i} = Linear( Concat( e(s^1), e(s^2), e(s^3) ) )
• GRPO-SR Loss: L_{GRPO-SR}(θ) = L_{policy} + α · L_{nll}

These need to appear right after explanatory text, not just at document end.

Current Workaround

Users must manually insert formulas via Feishu desktop client, breaking API-first workflow.

Start by locating the implementations of docs +media-insert and docs +update --mode insert_after, then inspect how document blocks are sent to the Feishu API. Done means positional insertion works with media and LaTeX equation blocks, including --index and insert_after, without requiring the desktop client.