perf(tm2/bptree): find the oldest and newest version with two seeks

[davd-gzl]

Follow-up to #5937.

Opening the store reads every stored version to find just the oldest and the newest. gno.land keeps 705,600 of them, and every query at a height pays for it.

Version keys are already sorted: R followed by the version as 8 bytes, biggest part first.

R…0003   ← oldest
R…0004
R…0005
  ⋮
R…AC44   ← newest

Before, per query:

read R…0003, R…0004, R…0005 … R…AC44     705,600 reads
track the smallest and largest seen

After:

seek to the first R key    → oldest
seek to the last R key     → newest      2 reads

The values were always at the ends; the scan just never looked there. A new test covers the case where pruning removes the oldest versions, so the first key is no longer version 1.

Opening the store this way can also write to the database, since loading rebuilds a stale lookup index. Left for a follow-up, written up in the ADR: skipping the rebuild alone is unsafe, because a read trusts an index entry on its version alone and a stale index would return wrong values.

[Gno2D2]

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