test: characterize transcoding contracts and models

[jahvari]

Summary

• characterize the existing HLS, casting, device-info, profiler, and transcode-profile compatibility contracts
• add closed, validated transcoding domain types without routing production playback through them yet
• add a deterministic cross-platform fake FFmpeg/ffprobe-style media tool for later lifecycle and codec tests

Scope

This is PR 1 of the hardware-transcoding implementation sequence. It is deliberately observational and foundational: it locks current public behavior and introduces the type/test seams required by later runtime, probing, planning, and hardware-backend PRs. It does not enable the new hardware-transcoding path.

The follow-up transcoding/02-jellyfin-runtime branch is stacked on this exact commit history, so this PR should use a normal merge commit rather than squash or rebase merge.

Verification

• cargo fmt --all -- --check
• cargo clippy -p fake-media-tool --all-targets -- -D warnings
• cargo clippy -p server --no-default-features --features librqbit --all-targets -- -D warnings
• cargo test -p fake-media-tool — 27 passed
• cargo test -p server --no-default-features --features librqbit -- --test-threads=1 — 81 passed, 2 intentionally ignored characterization/download tests
• public-diff scan: no credentials, private keys, personal paths, local memory/docs, debug logs, or binary/archive additions

Follow-up

After this merges and CI is green, the Jellyfin FFmpeg runtime-foundation PR will be retargeted to master so its diff contains only Tasks 4–6.