explaining that Brady is not violated when appellant knew, or with reasonable diligence could have uncovered, evidence in question, or when evidence was available to defense from other sources

explaining petitioner offered no affidavit or other evidence as to what trial counsel did or did not investigate; petitioner provided no explanation for absence of affidavit from trial counsel and proffered no evidence as to what actions trial counsel took or failed to take; thus, any assertion that trial counsel had no reasonable basis for failing to recognize or present evidence of alternative suspect was speculative