Add configurable static CT tail start overlap

[auspexlabs]

Apologies for the earlier PR on this. I closed the first attempt because it was premature; this is the smaller focused patch after letting the fork bake in live operation longer.

This makes the fresh-start overlap for static CT watchers configurable while preserving the current default behavior.

Today a new static CT watcher starts at tree_size - 256. This keeps 256 as the default, but exposes it as ct_log.start_overlap_leaves with an env override for operators that want a wider or narrower tail window.

Changes:

• add ct_log.start_overlap_leaves
• add CERTSTREAM_CT_LOG_START_OVERLAP_LEAVES
• bound the value during config validation
• keep existing 256 behavior as the default

Validation:

• cargo test
• git diff --check