feat: rebrand to MMD PDF, single corporate theme, and remove original text under an edit

- Services/PdfRedactText.cs: strip text whose origin falls inside a CoverAnnotation
  from the page content stream at save time, hooked into PdfBurn.DrawAnnotationsIntoDoc.
  Fixes edited values staying recoverable by text extraction.
- Themes/MMD.xaml replaces all thirteen themes; picker and accent strip removed; no dark mode.
- Rename KillerPDF -> MMD PDF across code, resources, packaging and locale strings; new icon.
- Remove the upstream author credit and the in-app install button.

validation/Compare-VeraPDF.ps1

@@ -4,12 +4,12 @@
     validation outcome changed between the two runs.
 
 .DESCRIPTION
-    Used to verify that resaving PDFs through KillerPDF does not degrade
+    Used to verify that resaving PDFs through MmdPdf does not degrade
     standards conformance. Workflow:
 
       1. Baseline the corpus:
          verapdf --recurse --format json C:\pdf-corpus > baseline.json
-      2. Resave every corpus file through KillerPDF into a mirror folder,
+      2. Resave every corpus file through MmdPdf into a mirror folder,
          preserving relative paths (see --batch-resave).
       3. Validate the resaved tree:
          verapdf --recurse --format json C:\pdf-corpus-resaved > after.json
@@ -29,7 +29,7 @@
 
 .NOTES
     Compatible with Windows PowerShell 5.1 and PowerShell 7.
-    Part of the KillerPDF validation harness (validation/).
+    Part of the MmdPdf validation harness (validation/).
 #>
 [CmdletBinding()]
 param(
@@ -233,7 +233,7 @@ if ($CsvOut) {
 }
 
 if ($regressionCount -gt 0) {
-    Write-Host 'RESULT: FAIL - KillerPDF resave introduced regressions.' -ForegroundColor Red
+    Write-Host 'RESULT: FAIL - MmdPdf resave introduced regressions.' -ForegroundColor Red
     exit 1
 } else {
     Write-Host 'RESULT: PASS - no conformance regressions introduced.' -ForegroundColor Green