Add offline end-to-end tests for watchdogDownloader

[Whning0513]

Problem

The current CI checks wdd with bash -n and --version, but it does not exercise project initialization, resumed HTTP downloads, watchdog state, or manifest verification. Regressions in those paths could pass CI.

Goal

Add deterministic, offline end-to-end coverage for watchdogDownloader/wdd on Linux. The test must use temporary directories and a local HTTP server; it must not depend on a public download URL.

Acceptance criteria

• Start a local test server that serves a deterministic binary fixture and supports byte-range requests.
• Generate a TSV manifest with exact byte size and SHA-256 values.
• Exercise wdd init and a foreground download path in a temporary project.
• Pre-create a partial output and verify that the next request resumes it rather than replacing valid bytes.
• Verify that wdd verify succeeds for the completed fixture and returns non-zero for a truncated or corrupted fixture.
• Clean up the HTTP server, downloader processes, locks, and temporary files even when an assertion fails.
• Run without external network access and avoid long fixed sleeps.
• Add the test to .github/workflows/ci.yml.

A plain Bash test is sufficient; a testing framework is optional. Keep production defaults and manifest compatibility unchanged unless the test exposes a real bug.