Pure, deterministic label generator used at build time by
 `scripts/gen-unit-labels.fsx` to produce `UnitLabels.generated.fs`.
 Feature 028-unit-viz-language.

 Unit labels are partitioned by `(MovementShape, FactionId)` and only
 required to be unique *within* that bucket — the rendered glyph's
 shape + stroke colour already distinguishes factions and movement
 types, so a one-glyph label reads unambiguously alongside them.

 Allocation order per bucket:
   1. Preserve any label from `previous` that is still unique within
      the bucket (SC-006 stability).
   2. For each remaining name, propose a single-character label
      derived from the internal name (first consonant → any letter
      → name digit), then sweep the global single-char pool (upper
      Latin → lower Latin → digits → unique Greek letters).
   3. When the single-char pool is exhausted in a bucket, fall back
      to the two-char pool (`Aa` style) using the same name-derived
      → exhaustive sweep policy. Only the densest shape+faction
      tails of `BarData` should ever reach this path.