feat(lfs): add FastCDC media transport

[marshawcoco]

Summary

• connect Libra's existing frozen fastcdc-v1 chunker, manifest, and local chunk store to normal LFS push/download flows
• negotiate the repository-scoped Mega media extension while preserving standard LFS fallback before a chunked transfer is selected
• upload only missing chunks, resume interrupted transfers, reuse verified cached chunks, and repair corrupt cache entries
• fail closed on authentication or integrity errors after selecting a finalized manifest
• derive repository-scoped LFS URLs without dropping paths, query strings, ports, or IPv6 hosts
• atomically publish hydrated files after whole-object verification
• flush ordinary LFS downloads and checksum-error pointers before returning, fixing an observed Tokio write-completion race
• document the opt-in configuration, authentication flow, behavior, and known production gaps

This is the client half of a coordinated Mega server change. Mega companion PR: gitmono-dev/mega#2178

Verification

• cargo test --features fastcdc --test compat_fastcdc_feature_gate_guard: 2 passed
• cargo test --features fastcdc --test media_fastcdc_test: 6 passed, 1 interop case ignored
• cargo test --features fastcdc --lib utils::media::: 12 passed
• default and FastCDC checks passed; the FastCDC release build passed on Windows/MSVC
• the ignored Libra-to-Mega HTTP case passed against the companion Mega production routes and test fixture
• cargo +nightly fmt --all --check, git diff --check, and a Playwright 1.62.1 Chromium install dry-run passed after repairing the self-hosted Web E2E workflow

The PR also removes Playwright's per-job --with-deps invocation from the self-hosted Web E2E job. The runner must provide its Linux browser dependencies; the old command attempted interactive sudo and had timed out for five minutes across multiple unrelated PRs before any E2E test ran.

cargo test --features fastcdc --lib completed with 4,196 passed and 98 failures in non-FastCDC Windows-sensitive areas. Those failures were not individually proven to be baseline failures because the clean-baseline comparison was itself blocked by unguarded Unix-only test code. The two --all runs and strict all-targets/all-features Clippy were also non-zero on Windows. This PR does not claim full CI is green.

Deployment boundary

The feature is disabled by default and can be disabled per repository with lfs.fastcdc=false. The HTTP fixture uses loopback, an injected test client/token, SQLite, and Local object storage. It does not validate production credential storage, a complete Mono deployment, S3/GCS backends, shared repository ACLs, quotas, or garbage collection.