feat: placeholder normalization and validation in library

[WendellXY]

Adds placeholder parsing, normalization (%@ → %s, %1 → %1, %ld/%lu → %d/%u) and validation APIs to the lib. Provides optional normalization in conversions, tests, and docs examples. Updates ROADMAP accordingly.

[Copilot]

Pull Request Overview

Adds comprehensive placeholder normalization and validation functionality to the langcodec library. This enables cross-platform compatibility by normalizing iOS placeholder formats (%@, %1$@, %ld) to canonical Android-compatible forms (%s, %1$s, %d/%u) and provides validation to ensure placeholder consistency across languages.

• Introduces a new placeholder module with parsing, normalization and signature extraction capabilities
• Adds optional normalization to conversion functions with new convert_with_normalization and convert_auto_with_normalization APIs
• Implements placeholder validation methods in the Codec for detecting cross-language inconsistencies

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
langcodec/src/placeholder.rs	New module containing placeholder parsing, normalization, and signature generation logic
langcodec/src/lib.rs	Exports new placeholder functions and updated conversion APIs
langcodec/src/converter.rs	Adds normalization variants of convert functions and comprehensive test coverage
langcodec/src/codec.rs	Implements placeholder validation and normalization methods in the Codec struct
ROADMAP.md	Updates milestone status to reflect completed placeholder normalization feature

Comments suppressed due to low confidence (2)

langcodec/src/converter.rs:282

• This helper closure is duplicated between the original convert function and convert_with_normalization. Consider extracting it to a shared helper function to reduce code duplication.

    let pick_resource = |lang: Option<String>| -> Option<Resource> {
        match lang {
            Some(l) => resources.iter().find(|r| r.metadata.language == l).cloned(),
            None => resources.first().cloned(),
        }
    };

langcodec/src/converter.rs:1

• The placeholder validation logic in validate_placeholders and collect_placeholder_issues is nearly identical. Consider extracting the common logic into a shared helper function to reduce duplication.

//! Format conversion utilities for langcodec.

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