fix(upgrade): remove hard chain depth cap for nightly delta upgrades (#444)

The `MAX_CHAIN_DEPTH=10` limit silently rejected nightly delta upgrade
chains
when >10 commits landed on `main` between the current and target
versions (~2-3
days of normal development). This caused fallback to full binary
downloads (~30 MB)
even when the total patch chain was much smaller.

The existing `SIZE_THRESHOLD_RATIO=0.6` already guards against chains
where cumulative
patch size exceeds 60% of the full binary — this is the meaningful
cost/benefit check.
The hard depth limit was redundant for nightly and fired before the size
check could run.

### Changes

- Rename `MAX_CHAIN_DEPTH` → `MAX_STABLE_CHAIN_DEPTH` (stable keeps the
limit since
  it also controls GitHub Releases `per_page`)
- Remove hard depth cap from `resolveNightlyChain` — nightly chains now
rely solely on
  the size budget
- Add debug logging when chain resolution is rejected so `--verbose`
reveals why delta
  was skipped

Author: BYK