Fix SetText replacement after leading comments

[fhgffy]

When an element starts with comments, SetText() currently inserts a new text node before the comments and leaves the previous value in the tree. For <foo><!--comment-->old</foo>, setting "new" serializes as <foo>new<!--comment-->old</foo>.

Skip leading comments when selecting the text node, matching GetText(), and replace that existing node. Keep first-child insertion when no text node follows the comments. Add regression coverage for consecutive comments, mixed content, CDATA, numeric setters, and the insertion fallback, and update the SetText() documentation.

Fixes #927.

Validation on Windows with GCC 15.1.0, CMake 4.3.2, and Ninja 1.13.2:

• The minimal reproducer fails on master 8224e427b655b83dae5e2298f1e6919523a78737 (exit 1) and passes with the fix (exit 0).
• The original suite passes 528 checks. With only the regression tests added, 12 checks fail; after the fix, all 558 checks pass in Debug and Release.
• CMake/Ninja static/shared Debug/Release builds and CTest all pass. The local MinGW Makefiles generator failed to handle the workspace's Unicode absolute path; Ninja was used for the CMake checks.