Add per-module log level filtering

[oubiwann]

Summary

• Add module_filters field to Opts and OptsBuilder for per-module log level overrides
• Module filters match log record targets by prefix; first matching prefix wins
• Unmatched modules fall back to the global level
• log::set_max_level() set to the most permissive level across global + all filters so messages reach the custom enabled() check

Example usage

let opts = OptsBuilder::new()
    .level(LogLevel::Trace)
    .module_filter("tokenizers", LogLevel::Warn)
    .module_filter("hyper", LogLevel::Info)
    .build()
    .unwrap();

This suppresses tokenizers TRACE/DEBUG/INFO while keeping TRACE for everything else.

Files changed

File	Changes
src/opts.rs	module_filters field on Opts + OptsBuilder, accessor, builder methods, serde support, 5 new tests
src/logger.rs	module_filters on LoggerConfig, per-module filtering in enabled(), effective max level in dispatch(), 5 new tests
src/lib.rs	Updated setup() doc comment with module_filters example

Test plan

• All 156 unit tests pass
• All 15 integration tests pass
• All 12 doc tests pass (2 new)
• cargo clippy -- -D warnings clean
• cargo fmt --check clean
• Manual verification with a program that sets level=Trace, module_filters=[("tokenizers", Warn)]

Closes #8

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