Repair missing Windows curl automatically and improve diagnostics

[KBRASK]

Changes

• Find and check Windows system, PATH, Git and managed curl copies before setup.
• Automatically obtain a pinned, checksum-verified curl only when no usable copy exists, without changing system PATH or enlarging the launcher with a bundled binary.
• Keep the selected proxy/direct policy during repair and report unsupported bootstrap proxy configurations explicitly.
• Include discovery results, Windows exit codes, repair route, timing and integrity failures in local redacted diagnostic reports.
• Show Windows-specific bilingual recovery guidance instead of instructions to run setup.sh.

Validation

• Python/JSON syntax and patch whitespace checked.
• Pinned archive and member hashes obtained from the official curl Windows distribution.
• Installation changes exported from committed source only.
• Native Windows execution and automated runtime tests have not been run for this change.