fix(promote): build channel-aware delta when promoting across skipped versions

[peters]

Summary

surge pack always builds the release delta against the immediately-previous version overall, regardless of channel. When intermediate releases live only on test between two production releases, the production-side delta basis no longer matches what production nodes have on disk. The first per-file PatchFile basis-hash check during apply fails:

```
Sparse delta file hash mismatch for 'camera-tuner.deps.json':
expected , got
```

The supervisor then never activates the delta. The only operational workaround has been `surge compact ` after every promotion, which deletes the broken delta and forces every fleet node into a full reinstall. This pattern shows up in every production promotion log of the youpark fleet (Apr 10, Apr 11, Apr 12).

What changes

• commands/promote.rs — after `ensure_release_full_artifact`:
  1. Look up the previous release on the target channel.
  2. If the release does not already carry a delta with `from_version` matching that predecessor, restore both full archives, build a sparse delta, upload it as `---from--delta.tar.zst`, and `upsert_delta` it onto the release entry.
  3. Original test-channel delta is preserved (multi-delta), so test chains keep working.
  4. Re-running `surge promote` on an already-promoted release also rebuilds missing channel deltas, so releases promoted before this fix can recover without a demote/repromote cycle.
• update/manager/release_index.rs — chain walker resolves each apply step by `delta_from_source(prev_step.version)`. Releases pushed before `from_version` was tracked carry one delta with an empty `from_version`; the resolver accepts those as a legacy fallback so existing chains keep working. Returns `None` (forcing full install) when no compatible delta exists, instead of letting the apply path crash with the basis-hash mismatch.
• releases/manifest.rs — adds `ReleaseEntry::delta_from_source` and `ReleaseEntry::upsert_delta` helpers.

Test plan

• `./scripts/sync-surge-core-vendor.sh --check`
• `./scripts/check-version-sync.sh`
• `cargo fmt --all -- --check`
• `RUSTFLAGS="-D warnings" cargo test --workspace` (358 tests, +4 new)
• `cargo clippy --workspace --all-targets --all-features -- -D warnings`
• `cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used`
• `cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic`

New tests

• `commands::promote::tests::execute_rebuilds_channel_delta_when_promoting_across_skipped_versions` — constructs real archives via `ArchivePacker`, sets up the `v1.0.0(prod)` / `v1.1.0(test)` / `v1.2.0(test)` layout from the field bug, promotes `v1.2.0` to production, and verifies the rebuilt delta correctly transforms a v1.0.0 archive into v1.2.0 with the new `camera-tuner.deps.json` content.
• `update::manager::release_index::tests::resolver_picks_delta_matching_current_install_version`
• `update::manager::release_index::tests::resolver_falls_back_to_legacy_delta_with_empty_from_version`
• `update::manager::release_index::tests::resolver_returns_none_when_no_delta_matches_or_is_legacy`