feat: line badges, rarity tags, dynamic masthead

[Calixteair]

What

Three visual improvements to make the game more readable and a bit more rewarding to play.

1. RATP line badges — predicate labels like "Sur la ligne 7" now inline a colored pastille for the line number instead of leaving it as plain text. M1..M14 render as discs, RER A..E as rounded squares.
2. Live rarity tags — every time the player answers correctly, the server returns the entity's fame_score. The client derives a Common / Rare / Épique / Légendaire tier (80/50/20 thresholds), pops a transient toast above the grid, and leaves a persistent corner badge on the cell.
3. Dynamic masthead — the header eyebrow + title + date now reflect the current mode (daily / solo / duel) and domain (Métro / RER) instead of hardcoding "Grille du jour / Métro de Paris" everywhere.

Why

The masthead bug was an explicit ask after the multi-domain refactor — /rer/play was still claiming "Métro de Paris" in its header, which is obviously wrong.

Line badges and rarity tags came together: both lift the UI from "list of grey predicates" to something that feels like the metro/rer it represents. With #57 baking real fame scores into the dataset, the rarity dimension can finally surface during play instead of waiting for the end-game modal.

How

40771a5 feat(web): inline SVG line badges in predicate chips

• New LineBadge.svelte component. Inline SVG (no binary assets), colors from the official RATP palette via CSS variables. Discs for métro, rounded squares for RER. Scales with size prop (default 18 px).
• PredicateChip.svelte tokenises the localised label with a regex (/\b(?:ligne|line|RER)\s+([A-E]|\d{1,2}(?:\s?bis)?)\b/). Tokens render text or badge. Other predicate families untouched.
• Grid forwards its existing domain prop down so the chip picks the right palette.
• Editorial use only — disc + line code, no RATP wordmark.

1efad9c feat: live rarity tags + dynamic masthead

Contracts + server:

• PlayResponse carries an optional fameScore field. Server reads grid.payload.entities[id].fame_score (snapshotted at generation time, stable through re-ingest), null on wrong answers and un-scored entities.

Web — rarity:

• src/lib/rarity.ts: rarityFor(fame) with 80/50/20 thresholds, null → 'common'.
• CellAnswer.fameScore persisted in localStorage so badges survive a refresh.
• CellButton.svelte: persistent corner badge on cells where rarity ≥ rare (common cells keep the understated check).
• Grid.svelte: transient toast above the grid on each correct answer ≥ rare, 1.7 s fade, gold w/ glow for legendary. prefers-reduced-motion respected.

Web — masthead:

• Grid header reads mode + domain props. Eyebrow flips between daily / solo / duel labels. Title flips too. Date renders only in daily mode (solo/duel grids aren't "of the day"). domainLabel switches between paris-metro and rer i18n keys.

i18n: 13 new fr+en keys (4 rarity tiers, grid_mode_, grid_title_).

Checklist

• Conventional commit titles, no AI / Claude mention
• cargo fmt --check && clippy --all-targets -- -D warnings && test --workspace passes
• pnpm --dir web lint && typecheck && test && build passes
• Mobile-first respected (badges scale, rarity toast positions over the grid, masthead unchanged at 360 px)
• No new unsafe, no unwrap() in non-test code
• No new secret
• Contract change in contracts/openapi.yaml (additive — fameScore is optional)

Test plan

• Open /paris-metro/play, answer with Châtelet → toast doesn't show (common), no badge
• Answer with a fame=15 station → 'Légendaire' toast pops, gold badge stays in the corner after toast fades
• Refresh mid-game → badges still there
• Predicate "Sur la ligne 7" → '7' renders as a pink disc
• Open /rer/play → predicate "Sur la ligne A" → 'A' renders as a red rounded square
• Masthead on /paris-metro/play shows "Grille solo / Métro de Paris" + title "Solo", no date
• Masthead on /rer/ (daily) shows "Grille du jour / RER d'Île-de-France" + title "Grille du jour" + today's date