fix: Fix panic on headers with no text content

[Flamenco]

Fixes a panic when processing headers that contain only attributes (e.g., # {#custom-id}) or other non-text content.

Problem:
Headers with no Text or Code events would leave the span end at 0 while the start was a positive offset, causing a slice panic: begin <= end (16 <= 0) when slicing.

This can happen when an include file contains and the document has headers with only custom ID attributes.

Solution:

• Track header span end as Option instead of usize
• Skip headers where no text content was found

Includes a regression test.

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