Stop dropping content from pre blocks

[trsdn]

Fixes #19.

Stacked on #29 (which is stacked on #28). Review those first.

Problem

The <pre> branch resolved a single direct <code> child and converted only that node:

var codeNode = node.SelectSingleNode("code");
ConvertChildren(codeNode ?? node, sb, listDepth, 0, true);

Anything else in the block was silently discarded.

Change

• Language hint is taken from the first descendant <code> (.//code), so wrapped markup is still recognised.
• The whole <pre> subtree is converted, so no children are dropped.
• The fence is sized to be longer than the longest backtick run in the content, instead of always three.

Verification

Input	Before	After
<pre><code>first</code>\n<code>second</code></pre>	first only — second lost	first + second
<pre><div><code class="language-py">print(1)</code></div></pre>	no language, stray blank line	```py + print(1)
<pre><code> containing ```	3-backtick fence → broken nested fence	4-backtick fence
<pre><code class="language-cs">…</code></pre>	correct	unchanged
<pre>raw text</pre> (no <code>)	correct	unchanged

dotnet build -c Release -r win-x64 succeeds with 0 warnings. The ad-hoc Regex.Match for the language class is now a source-generated regex, consistent with the rest of the file.