PHLEGM v9 Revision r7 - Windows PCRE2 download fallback
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Source: PHLEGM v9 r6 + compiler log PHLEGM-v9-Compiler-Logs-2026-1012-1721-3257.zip

Problem: Three of four Windows targets compiled. Windows 64bit OpenGL
failed before compilation when the MSYS2 PCRE2 download endpoint returned
HTTP 403 Forbidden. The compiler's aggregate exit code was 1.

Changes:
- Prefer MSYS2 primary repo.msys2.org rather than geo-redirector first.
- Fall back to the official listed UMD Tier-1 MSYS2 mirror and then the
  geo-redirecting mirror.msys2.org if primary cannot be accessed.
- Every downloaded package MUST match its pinned SHA-256 digest; the digest
  is verified independently for every candidate source.
- Downloads are written to unique .part files, never treated as cached until
  digest verification succeeds, and deleted after any failure.
- Reuse SHA-256 verified cached packages, even without network connectivity.
- Optionally seed an offline runtime from a system MSYS2 installation
  (C:/msys64 or C:/tools/msys64, or MSYS2_ROOT), but only with the matching
  PE architecture and PCRE2 redistribution license.
- Retain PE-machine checks after extraction and architecture-specific DLL names.
- No changes to regex.lib, aa.lib, embedded FBVM, Linux PCRE2, or microcode.
- No extra Python files shipped; original shared Compiler UI/bootstrap scripts
  remain unchanged.

Validation boundaries:
- Four Windows compile target results from user's r6 logs: 3 PASS, 1 FAIL.
- Source and manifest were statically inspected in this environment.
- Windows PowerShell 5.1, live MSYS2 network failover, PCRE2 Win32/Win64
  binary loading, and full native builds must be tested on the user's machine.
- An HTTP 403 from ALL trusted mirrors remains an explicit build failure,
  NOT success. Offline packages/DLLs remain supported.
