refactor: deduplicate ListedLogFiles / LogSegment and clean up validation (delta-io#1983)

## What changes are proposed in this pull request?

This PR cleans up the relationship between `ListedLogFiles` (now
  `LogSegmentFiles`) and `LogSegment`, removing several sources of
  duplication:

  - **Remove `ListedLogFilesBuilder`**: it was an exact structural
    duplicate of `ListedLogFiles`. Replaced with `#[derive(Default)]`
    and direct struct-literal construction + `..Default::default()`.

  - **Rename `ListedLogFiles` → `LogSegmentFiles`**: the struct combines
    filesystem-listed files _and_ catalog-provided log tail commits, so
    "listed" was a misnomer. Also renames the module.

  - **Embed `LogSegmentFiles` in `LogSegment`**: `LogSegment` previously
    declared the same 6 fields (`ascending_commit_files`,
    `ascending_compaction_files`, `checkpoint_parts`, `latest_crc_file`,
`latest_commit_file`, `max_published_version`) that `LogSegmentFiles`
    already had. These are now accessed via `log_segment.listed.*`.
    Also removes `into_parts()` since `try_new()` now mutates the struct
    in place.

  - **Consolidate validation into `LogSegment::try_new`**: every
    `LogSegmentFiles` construction flows into `try_new`, so `validate()`
    on `LogSegmentFiles` was a redundant pre-check. The structural
    invariant checks (compaction sort order, checkpoint integrity) are
    now a `#[cfg(debug_assertions)]` block at the top of `try_new`,
    alongside the existing always-on checks (contiguity, checkpoint gap,
    end version).

Follow up PR that consolidates all the validations to a single loop and
enables them for prod.
delta-io#1995

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## How was this change tested?
unit test added

---------

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: chiinlquah

kernel/src/lib.rs

@@ -147,7 +147,7 @@ pub mod last_checkpoint_hint;
 #[cfg(not(feature = "internal-api"))]
 pub(crate) mod last_checkpoint_hint;
 
-pub(crate) mod listed_log_files;
+pub(crate) mod log_segment_files;
 
 #[cfg(feature = "internal-api")]
 pub mod history_manager;

kernel/src/log_reader/checkpoint_manifest.rs

@@ -136,8 +136,8 @@ mod tests {
     ) -> DeltaResult<()> {
         let log_segment = snapshot.log_segment();
         let log_root = log_segment.log_root.clone();
-        assert_eq!(log_segment.checkpoint_parts.len(), 1);
-        let checkpoint_file = &log_segment.checkpoint_parts[0];
+        assert_eq!(log_segment.listed.checkpoint_parts.len(), 1);
+        let checkpoint_file = &log_segment.listed.checkpoint_parts[0];
         let mut manifest_phase =
             CheckpointManifestReader::try_new(engine.clone(), checkpoint_file, log_root)?;
 
@@ -218,7 +218,7 @@ mod tests {
 
         let manifest_phase = CheckpointManifestReader::try_new(
             engine.clone(),
-            &snapshot.log_segment().checkpoint_parts[0],
+            &snapshot.log_segment().listed.checkpoint_parts[0],
             snapshot.log_segment().log_root.clone(),
         )?;