Simplify cross-launch media process recovery

[thsnkhn]

Objective

Simplify media process recovery by supervising yt-dlp during the current Harbor session only.

Complete #65 before this work.

Current problem

MediaDownloadService persists PID, process-group, executable, command, start-signature, and recovery-directory ownership. Startup scans the process table, validates exact identities, terminates verified orphan groups, preserves unverifiable markers, and recovers child completion receipts.

This protects rare app-crash windows but adds a large cross-launch process-management subsystem.

Proposed design

• Keep ManagedChildProcess supervision while Harbor is running.
• Terminate active media processes during normal pause, cancel, removal, and shutdown.
• Keep a simple per-attempt media recovery manifest and output receipt.
• On startup, recover completed output when evidence is valid.
• Treat old active attempt directories as interrupted.
• Use best-effort cleanup instead of inspecting or killing processes from an earlier Harbor process.

Remove:

• cross-launch PID and PGID ownership manifests
• process-table enumeration
• executable and start-signature validation
• orphan process-group termination
• marker quarantine and stale-owner barriers
• child monitor machinery used only for cross-launch ownership guarantees

Accepted edge cases

• After Harbor crashes, yt-dlp or a converter can continue until it exits.
• A relaunch does not kill a process started by the previous Harbor instance.
• A retry can require manual cleanup if the old process still owns the output.
• Multiple Harbor instances are not supported.

Safeguards to retain

• Supervision and cancellation during a normal Harbor session.
• Attempt identifiers for stale events.
• Destination containment and regular-file validation.
• Media partial-file resume when the recovery directory is usable.
• Normal shutdown cleanup.

Acceptance criteria

• Normal media start, progress, pause, resume, cancel, completion, and shutdown work.
• Valid completed output can still be recovered after relaunch.
• Harbor never kills an unverified external process.
• Cross-launch process ownership code is removed.
• Production code decreases by approximately 400 to 700 lines.
• Focused media runtime and recovery tests pass.