fix: MAGA (Make Argentina Great Again)

[lorenzotomasdiez]

What

MAGA: Make Argentina Great Again.

Adds an optional scalar multiplier that boosts posts of Argentine origin, applied alongside the existing author-diversity and out-of-network multipliers.

Two new feature switches, both off / no-op by default:

• rust_home_mixer_enable_maga_boost (bool, default false)
• rust_home_mixer_maga_boost_factor (f64, default 1.15)

How

PostCandidate carries no author-country signal, so the boost keys off the post itself (home-mixer/scorers/maga.rs):

• the Argentine flag emoji, accepted in any language
• a short list of high-signal markers - che, boludo, mate, asado, milei, messi, maradona, quilombo, argentina, el mejor pais del mundo - but only for posts already tagged language_code == "es"

Text is normalized first: lowercased, diacritics folded, and everything non-alphanumeric collapsed to single spaces. Markers are then matched on word boundaries, which gives three properties worth calling out:

1. Substrings do not match. che inside noche / leche and mate inside matemática are rejected.
2. Accents and punctuation do not matter. ¡MILEI! and el mejor país del mundo both match.
3. Multi-word markers work the same way as single words, so the list can hold phrases without a separate code path.

Markers that read as ordinary words elsewhere (mate, messi, argentina) are gated behind the Spanish tag, so "thanks mate" does not match.

Wired into RankingScorer::score at the point where effective_oon and the diversity multipliers are already combined, in both the MPN path and the weighted path, so the two stay consistent.

Tests

• 9 unit tests on the detector in maga.rs (single-word and multi-word markers, accent and punctuation folding, flag-in-any-language, whole-word matching, language gating, unmarked Spanish, untagged language, empty text)
• 2 tests in ranking_scorer.rs: boost applies at the configured factor with the switch on, and is an exact no-op with it off