v0.27.1 — review fixes: continuous rivers, robust config, perf

Bug fixes, cleanup, and a byte-identical performance win from a full-codebase
review. One behaviour change: rivers are now continuous (see Fixed).

Fixed

• Rivers are continuous source→mouth polylines again. They were traced
  highest-flux-first, which shattered every river into 2-cell fragments — harmless
  on the rendered map (the pieces tile), but TerrainResult.rivers and its
  serialisation were fragmented, with per-fragment widths. Now traced
  headwaters-first, and River.width comes from the downstream (mouth) end.
  This changes rivers output for a given seed.
• WorldMapConfig.from_dict no longer crashes on noisy payloads. A str,
  None, NaN, or inf value now coerces to a finite number (or the field
  default) and clamps, instead of raising TypeError — honouring the documented
  "safe for sloppy LLM/JSON input" contract.
• edge_falloff was applied squared in multi-continent worlds; now applied
  once. (Only affected continents>1 with edge_falloff≠1.0; no preset uses that.)
• Smart labels no longer misalign colour/style when a region or feature has an
  empty name, and features never get an empty name.

Changed

• Lloyd relaxation vectorised (np.bincount) — ~24% faster world generation,
  byte-identical output. Removed dead energy bookkeeping from the label placer
  (and made its inner loop scan only overlapping labels); de-cubed the dungeon
  extra-corridor loop.

Added

• to_json / from_json on Region, Road, CellSummary, and WorldMapConfig
  (parity with the other public dataclasses), and exported get_theme,
  theme_names, DEFAULT_THEME.