[CleanCut]

Fetch what staging a file on S3 reads on the async runtime, so a blocking thread no longer waits on S3 for it, except for images, video, audio, Parquet, and Arrow files, whose parsers seek. This does not affect local storage.

• Fetch a file's first bytes, the rows a CSV's schema is inferred from, and its line-count download on the runtime, with only the parsing on blocking threads.
• Stop reading a data frame's schema when its line count fails, and the reverse, so a failed count fails the inspection with the store's error.
• Fetch a JSON data frame whole on the runtime and parse it in memory, since Polars parses it whole.
• Fail staging with the store's error when the store returns no bytes partway through a file.
• Sniff a stored CSV or TSV's dialect for a data frame view through the same code staging uses.

ENG-2397

[coderabbitai]

📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • Improved CSV and TSV format detection, including sensible defaults when the format cannot be identified.
  • Improved row and column metadata for tabular files, including files stored in cloud storage.
  • Improved text line and character counts and metadata retrieval during file inspection.
  • Improved record counts for JSONL and NDJSON files while retaining format-specific size calculations for other supported file types.

Stored-file inspection and CSV schema reads now fetch data asynchronously. CSV dialect resolution is shared across readers and scanners. Tabular size calculation uses format-specific row-count paths, and workspace and S3 callers await stored inspection.

Changes: Stored-file metadata and tabular inspection

Layer / File(s)	Summary
CSV dialect and stored schema crates/liboxen/src/core/df/tabular.rs	CSV readers and scanners pass a CsvDialect. Stored CSV schema inference fetches a sniff sample and a bounded head asynchronously.
Asynchronous stored inspection crates/liboxen/src/repositories/metadata.rs, crates/liboxen/src/repositories/metadata/tabular.rs, crates/liboxen/src/repositories/metadata/text.rs, crates/liboxen/src/core/v_latest/workspaces/files.rs, crates/liboxen/src/storage/s3.rs, crates/liboxen/src/util/fs.rs, crates/liboxen/src/opts.rs, crates/liboxen/src/opts/count_lines_opts.rs	Stored-version inspection and its workspace and S3 callers are asynchronous. Line counting returns LineCounts; text and tabular metadata use counts and schema data. The CountLinesOpts API is removed. The S3 workspace test checks uploaded CSV metadata dimensions.
Format-specific size calculation crates/liboxen/src/core/df/tabular.rs	Size calculation reuses the scanned schema and selects height calculation by format.

Priority: Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Refactor

Sequence Diagram(s)

sequenceDiagram
  participant WorkspaceStaging
  participant inspect_stored
  participant StoredVersion
  participant stored_schema
  WorkspaceStaging->>inspect_stored: inspect stored version with optional line counts
  inspect_stored->>StoredVersion: fetch file head and missing line counts
  inspect_stored->>stored_schema: infer stored CSV schema
  stored_schema->>StoredVersion: fetch sniff sample and CSV head
  stored_schema-->>inspect_stored: return schema
  inspect_stored-->>WorkspaceStaging: return file metadata

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Possibly related PRs

• Oxen-AI/Oxen#1128: Adds StoredVersion inspection and schema paths that this change updates.
• Oxen-AI/Oxen#1113: Introduces InspectedVersionFile and the metadata inspection flow extended here.
• Oxen-AI/Oxen#1103: Changes reader-based metadata extraction and line counting used by this change.

Merge Risk: Low · up to a906e

A narrow stored CSV/TSV failure can be hidden as missing metadata; preserve the line-count error before merging.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly identifies the main change: moving stored-file byte and line-count fetching for staging onto the async runtime. It is specific and related to the changeset.
Description check		The description accurately covers the async S3 reads, parsing exceptions, failure handling, JSON processing, and CSV/TSV dialect sniffing implemented by the pull request.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Finishing Touches

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[Eric-Laurence]

Looks good

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @crates/liboxen/src/repositories/metadata.rs:
- Line 299: In StoredBytes::fetch_head, move the synchronous prefix-and-suffix
assembly into the blocking pool with the imported spawn_blocking function, await
its result, and keep updating the cached head with the joined bytes.

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• Repository: Oxen-AI/Oxen
• Branch: cleancut/s3-perf-38.3.1.11
• File: .coderabbit.yaml
• Commit: a346ce3
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Files selected for processing: 2

• crates/liboxen/src/core/df/tabular.rs
• crates/liboxen/src/repositories/metadata.rs

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