Fix shortcodes not resolving inside math expressions

[cderv]

When using shortcodes inside inline or display math (e.g., $5 + {{< meta five >}}$), the output contains hex-encoded UUID placeholders instead of the resolved shortcode value.

Root Cause

The two-stage shortcode encoding (introduced for Pandoc 3.7+ compatibility) converts shortcodes to UUID-guarded hex strings before pandoc.read(), then restores them afterward. The restoration walker in readqmd.lua and the shortcode resolution filter in shortcodes.lua both handle Code, RawInline, RawBlock, and Str nodes — but neither handles Math nodes. Since Pandoc parses $5 + UUID;HEX;$ into a Math node with the placeholder in its .text field, the shortcode is never restored or resolved.

Fix

Add Math node handlers to both pipeline stages:

1. readqmd.lua: restore hex-encoded shortcode text back to {{< ... >}} syntax (reuses existing unshortcode_text)
2. shortcodes.lua: resolve shortcodes to their values inside math text (reuses existing apply_code_shortcode)

Both inline ($...$) and display ($$...$$) math are covered by the single Math handler, since Pandoc uses one node type with a mathtype discriminator.

Fixes #14255