fix: protect page count alias from UBA reordering (issue #1925)

[prateek-dagar]

Changes
Replace the alias with a single placeholder PUA character (\uE000 with collision-safe scanning) before UBA bidi processing.
Because a single character is atomic, it survives UBA reordering intact at its correct semantic location.
Reconstruct the bidi fragments by expanding the PUA placeholder back into a TotalPagesSubstitutionFragment.

Fixes #1925

• A unit test is covering the code added / modified by this PR

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• A mention of the change is present in CHANGELOG.md

• This PR is ready to be merged

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[andersonhc]

Suggested change

	def test_alias_in_middle_with_shaping_markdown(tmp_path):
	pdf = fpdf.FPDF()
	pdf.add_font("Quicksand", style="", fname=HERE / "fonts" / "Quicksand-Regular.otf")
	pdf.add_font("Quicksand", style="B", fname=HERE / "fonts" / "Quicksand-Bold.otf")
	pdf.add_page()
	pdf.set_font("Quicksand", size=24)
	pdf.set_text_shaping(True)
	pdf.multi_cell(w=pdf.epw, text="**Pages {nb}** with shaping", markdown=True)
	assert_pdf_equal(pdf, HERE / "alias_in_middle_with_shaping_markdown.pdf", tmp_path)

Can you add a test to make sure it's not breaking markdown?

[prateek-dagar]

The test case test_alias_in_middle_with_shaping_markdown already exists in the test suite.

Additionally, I have added a new test case test_alias_in_rtl_text_markdown to cover markdown formatting with RTL text shaping.

[prateek-dagar]

Hi @andersonhc,

I updated the branch to fix the markdown formatting issue:

• Changed placeholder to \u001f (neutral character) to prevent UBA from splitting RTL runs and breaking markdown tags.
• Overrode page count fragment direction to LTR to prevent numbers from being reversed to "21" in RTL text.
• Added test_alias_in_rtl_text_rtl_alias to verify custom RTL script aliases shape correctly.

All tests are now passing!