Add sealed / active split to reconciliation report

[AnIrishDuck]

Create a new ReconcileReport struct to split out the stats. Then, never run any fixes on active (unsealed) segments. This is the first and largest half of getting reconciliation concurrent-ready. The other smaller half is properly interacting with retention.

Claude's summary (edited to remove irrelevant removed code):

Reconciliation previously ran every segment through the strict size-diff
pipeline, which flagged the currently active (writeable) tail segment of
every partition on every pass: writes accumulate on disk before the
manifest flush, so the active segment legitimately diverges. That noise
blocks running reconcile continuously alongside writes.

Split the output into two buckets keyed off each partition's sealed_ix
watermark, read once per partition pass:

• sealed bucket: segments at or below sealed_ix go through the existing
  strict-diff + CAS-fix pipeline, unchanged.
• active bucket: segments above sealed_ix (or all segments when the
  partition has never sealed one) are recorded informationally as
  { topic, partition, manifest_size, disk_size, delta } and never fixed.

Introduce ReconcileReport { sealed: ReconcileStats, active: Vec<..> } and
replace ReconcileJob::stats() with report(). The /info handler now also
returns the active-segment bucket (additive, serde-default field).

[JONBRWN]

get the rust check resolved but looks good