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

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 even 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 (depth exceeded,
  size budget blown, validation failure, no tags found) so --verbose
  reveals why delta was skipped

Author: BYK