Phase 15: Add HTTP compatibility matrix and integration test harness

[ashishsinghbora]

Goal

Prove the downloader works against real-world server behaviors instead of only ideal HTTP responses.

Scope

Create a local integration test harness covering:

• Range-supported server.
• No-range server.
• HEAD unsupported server.
• Redirects.
• Incorrect Content-Length.
• Invalid Content-Range.
• Connection drops mid-chunk.
• Slow responses.
• ETag/Last-Modified changes.
• HTTP 429/5xx retry behavior.
• Servers that ignore a requested range.

Acceptance criteria

• Test server scenarios are deterministic.
• Each scenario has an expected downloader behavior.
• Full-file hashes are compared after successful transfers.
• Regression tests are added whenever a production bug is fixed.