getTextFromXPointer returns a single text node, so a position on
<p><span>C</span>onvergence of Heaven and Earth</p> sent context 'C'.
The server text search matched the first 'C' in the chapter and stored
doc-start, reopening the web reader at the chapter beginning.
When the cleaned node text is tiny (<15 runes), walk up to 3 levels to
enclosing element pointers and return the first block text >=15 runes
(keeping the longest seen as fallback). Kindle-light: at most 3 extra
local reads, only on the tiny-text path, zero extra network. Char-offset
slicing applies to the original node text only, not parent blocks.
