Arrow: say the target's floor (step key floor)

[moust4ki]

In game, the arrow said "You're here" in the middle of a building while the NPC was one floor up. The client gives addons no height for a target, but the guides' notes already say the floor. This PR adds that as a step key and has the arrow say it in words.

What the player sees

• On arrival, on a step with floor, the distance line says "The NPC is one floor up" / "The NPC is at the top" / "The NPC is one floor down" instead of "You're here". For an object, a place or a monster it says "It's one floor up"... The arrow no longer fades after 3 s on such a step, because the player still has to find the stairs. If the previous step had already faded at the same spot, the arrow comes back.
• Within 30 yards, the detail line adds "One floor up" / "At the top" / "One floor down" (after the zone, joined with " · ", when a zone is shown).
• Only for the step's own targets. It is not shown for a corpse, a flight master or dock on the way, a persistent objective or a go point.
• 9 new strings in all 8 locales.

The key

• floor = "up" | "top" | "down". It is in the public contract (docs/guide-packs.md, docs/create-a-guide-with-ai.md), SPEC, Import.STEP_KEYS (the value is checked), tools/check_guides.lua (a bad value is an error; a note that names a floor its key does not say is a warning) and the example pack (Aldrassil trainer).
• tools/guide_floor.lua: an explicit table of English phrases plus an ambiguity list. A note that names two floors, or a floor that may not be the right one, gets no key. write_leveling.lua writes the key from the route notes, so the routes stay the single source. It never goes on a go step, because its point is the door and its note describes the next step.
• Leveling regenerated (write_leveling, then translate_guides). The diff was checked line by line: the new key is the only change, and no translation changed. Counts: up 279, top 73, down 60. Class quests, by hand: up 11, top 4, down 6.

Tests

• floor_test (10 tests): arrival with and without floor, the fade, the near-target hint, NPC vs object wording, the faded-then-floor case, locales, import, check_guides, the phrase table, and every Leveling or class step whose note names a floor carrying it.
• Smoke scenario: the Goldshire mage trainer, upstairs in the inn.
• tools/check_all.sh: ALL GREEN (81 files, 12 smoke runs). Rebased on the 6 / 25 / 40 arrival thresholds.

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