Optimize FileSource::get_path with single-pass string construction

[avalanche-staging]

Summary

Optimizes FileSource::get_path in intl/l10n/rust/l10nregistry-rs/src/source/mod.rs by replacing a multi-allocation string construction pattern with a single-pass approach.

Before

fn get_path(&self, locale: &LanguageIdentifier, resource_id: &ResourceId) -> String {
    format!(
        "{}{}",
        self.pre_path.replace("{locale}", &locale.to_string()),
        resource_id.value,
    )
}

This performs 3 heap allocations:

1. locale.to_string() — intermediate String
2. str::replace(...) — allocates a new String with StrSearcher overhead
3. format!(...) — final concatenation into yet another String

After

fn get_path(&self, locale: &LanguageIdentifier, resource_id: &ResourceId) -> String {
    use std::fmt::Write;
    const PLACEHOLDER: &str = "{locale}";
    let pre_path = &self.pre_path;

    let cap = pre_path.len() + resource_id.value.len() + 10;
    let mut result = String::with_capacity(cap);

    let mut last = 0;
    while let Some(pos) = pre_path[last..].find(PLACEHOLDER) {
        let abs_pos = last + pos;
        result.push_str(&pre_path[last..abs_pos]);
        write!(result, "{}", locale).unwrap();
        last = abs_pos + PLACEHOLDER.len();
    }
    result.push_str(&pre_path[last..]);
    result.push_str(&resource_id.value);
    result
}

This performs 1 allocation with 0 reallocs:

• String::with_capacity pre-allocates the buffer
• write!(result, "{}", locale) formats the locale directly into the buffer (no intermediate String)
• str::find replaces StrSearcher with a simpler substring search

Benchmark Results (simulation mode)

Source benchmarks show 35–45% instruction count reduction, with zero regressions on solver, registry, or preferences benchmarks:

Benchmark	Before (Ir)	After (Ir)	Change
browser/has_file	99,990	55,423	-44.6%
browser/fetch_file_sync	141,896	91,448	-35.6%
preferences/has_file	117,529	72,757	-38.1%
preferences/fetch_file_sync	187,290	120,258	-35.8%
simple/has_file	2,863	1,753	-38.7%
simple/fetch_file_sync	6,254	3,860	-38.3%

Other changes

• rustc-hash: Updated from v1 to v2 in l10nregistry-rs/Cargo.toml to resolve type conflicts with transitive dependencies (fluent-bundle, fluent-fallback)
• Benchmarking: Replaced criterion = "0.3" with codspeed-criterion-compat in dev-dependencies and added a CodSpeed CI workflow

[github-actions]

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