Refactor `cdel` DB from CesarSuber to CesarOnSuber for lexicographic key ordering

[jaelliot]

Background

Per Sam's review (2026-02-26 dev call), the cdel database in src/keri/db/basing.py (line ~1341) uses CesarSuber but should use CesarOnSuber.

# Current (basing.py)
self.cdel = subing.CesarSuber(db=self, subkey='cdel.', klas=coring.Diger)

Problem

The key space for cdel uses a sequence number (via Seqner.qb64/Number.qb64). When stored with a CESR code prefix, the base64 encoding is not guaranteed to be lexicographically ordered if the code ever changes. This means:

• Database iteration methods won't return entries in numerical order
• The behavior is fragile — changing from Seqner to Number without huge could silently break ordering

Proposed Fix

1. Change CesarSuber → CesarOnSuber for the cdel database
2. The key space would then use a hex string ordinal (no CESR code), guaranteeing lexicographic = numeric ordering
3. Callers would pass the integer sequence number directly instead of constructing a Seqner/Number instance for the key

Current callers in src/keri/app/delegating.py:

• Line ~96: cdiger = self.hby.db.cdel.get(keys=(prefixer.qb64, seqner.qb64))
• Line ~234: self.hby.db.cdel.put(keys=(pre, coring.Seqner(sn=serder.sn).qb64), val=coring.Diger(qb64=serder.said))

These would need to change to pass sn as an integer to the OnSuber-style key builder.

Context

• Related to issue Convert all ints JSON representations of sequence numbers for events to hex str or CESR us Number #420 (Seqner→Number conversions)
• PR Convert Seqner to Number for cdel #1247 addresses the Seqner→Number part of cdel but does NOT change the DB type to CesarOnSuber
• Sam noted this needs discussion with Phil before implementation
• This is a broader refactor than just Seqner→Number — it changes the DB storage model

References

• subing.CesarOnSuber — the target DB class (uses hex ordinals in key space)
• Sam's direction from 2026-02-26 dev call transcript

[KeaxD]

This issue should be closed, it was resolved in PR #1247