Add record field retention helpers

[weasel-lee]

Motivation

• Provide an API to filter record fields in-place without reallocating the record buffer via a retain method.
• Complement existing in-place trimming behavior so callers can both trim and filter fields efficiently.
• Keep UTF-8 correctness for StringRecord while reusing the byte-level representation for performance.

Description

• Added pub fn retain<F>(&mut self, keep: F) to ByteRecord that compacts fields in-place by copying kept fields and updating bounds.len and bounds.ends accordingly.
• Added pub fn retain<F>(&mut self, keep: F) to StringRecord which delegates to the underlying byte-level retain and converts slices to &str with unsafe { str::from_utf8_unchecked(...) } for predicate calls.
• Introduced/used trimming helper functions (trim_ascii_range and trim_unicode_range) and switched trimming implementations to operate in-place using copy_within and write indices.
• Added unit tests named retain_fields to both src/byte_record.rs and src/string_record.rs to verify retention behavior alongside existing trimming tests.

Testing

• Added unit tests retain_fields in src/byte_record.rs and src/string_record.rs covering typical retention scenarios.
• Existing unit tests for trim and other record invariants were left intact.
• No automated test run was executed as part of this change (cargo test was not run).
• CI/build status is not known from this rollout.

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Codex Task

[weasel-lee]

@codex

METADATA_JOB_REQUEST_V1
METADATA_ON_CREATE_SHA:33a3884d9c0bad505ad4d6e591de3b9399120314

Please update Rust metadata files for this PR. Submit the result directly to GitHub (commit) without asking for toggles or confirmations.

Rules:

• Allowlist: use glob patterns from .github/metadata/allowlist.txt (supports !exclude lines).
• Output path mapping: /path/to/src/a.rs -> /path/to/src/a.metadata.txt
• Prompt: follow .github/metadata/prompt.txt exactly for output structure and style.
• Added Rust file: generate metadata from Rust source + prompt.
• Modified Rust file: update metadata using Rust source + existing metadata + prompt.
• Deleted Rust file: if corresponding .metadata.txt exists, delete it.
• Skip Rust files < 100 lines; include list of skipped files in summary.

Targets:
Added:
(none)

Modified:

• src/byte_record.rs
• src/string_record.rs

Deleted:
(none)

[chatgpt-codex-connector]

Summary

• Updated ByteRecord metadata to capture in-place trim/retain compaction behavior, bounds invariants, and related risks. src/byte_record.metadata.txtL1-L57
• Updated StringRecord metadata to reflect in-place retain/trim behavior, unchecked UTF-8 assumptions, and extension points. src/string_record.metadata.txtL1-L55

Testing

• Not run (metadata-only change).

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