Implement Nebula sectors (Galactic Events expansion)

[guygranger]

Closes #303.

Adds full support for the Nebula sector mechanic from the Galactic Events expansion. Uses synthetic subsector position codes (295A/295B/295C) layered onto the existing single-position-string model — the parent hex acts as a render anchor and the three subsectors are exposed as first-class board entries that mirror the regular sector schema.

Highlights:

• helpers/NebulaHelper.py: subsector model, internal/external adjacency, parent neighbour-rewrite with rotation
• helpers/GamestateHelper.add_tile: branches on enable_nebulas to build parent + 3 subsector records, places ai-anc in the Ancient subsector
• helpers/GamestateHelper.setup_techs_and_outer_rim: shuffles 295 into ring II and 395 into ring III when nebulas are enabled (per rulebook p. 4 the ring-III tile is appended after the regular sector_draws)
• Buttons/Move.getTilesInRange: skips parent records, walks subsector adjacency including internal sibling hops (matches rulebook example of 'two Move Activations' to cross an internal wormhole)
• Buttons/Influence: areTwoTilesAdjacent returns True for sibling subsectors; getTilesToInfluence excludes any tile of type='nebula' (parent or subsector) — nebulas may never contain Influence Discs
• Buttons/Explore: handles the nebula-as-explore-source case via the parent-rewrite, and shows a one-line explanation when a nebula is placed (no influence prompt, no immediate discovery prompt)
• Buttons/DiscoveryTile._maybe_reroll_for_nebula: implements the Ancient Orbital / Ancient Monolith reroll rule — if either is drawn for a nebula subsector, shuffle it back and draw again before presenting the player's choice
• helpers/CombatHelper.claimNebulaDiscoveries + Turn.runUpkeep hook: end-of-Combat-Phase discovery resolution. A discovery in a nebula subsector is claimed only by a single non-AI player whose ships remain there at that point. Errors surface to the channel so a GM can recover via /tile resolve_specific_discovery_tile
• helpers/DrawHelper: subsector positions return None from board_tile_image (their ships are drawn on top of the parent's hex); _render_ships extracted so the parent hex can render all three subsectors' ships using each subsector's *_snap coords
• commands/setup_commands + setup/GameInit: enable_nebulas flag on /setup create_new_game (default True)
• tests/test_nebulas.py: 19 cases covering setup, three-subsector creation, disctile/ancient distribution, internal + external + rotated adjacency, the influence-target type filter, deck population with the flag on/off, and the orbital/monolith reroll edge cases

Verified subsector layout against printed art for both NGC 5189 (295) and NGC 1952 (395) — Ancient in the right subsector (B), Discoveries in the top (A) and bottom-left (C). All 23 tests pass.