Support locale-defined scales in intword

[SAY-5]

Fixes #181

Changes proposed in this pull request:

• Let translation catalogs define their own named powers of ten and pluralized number/unit patterns. Match each pattern table to its exact scale vector so an incomplete regional catalog cannot silently inherit labels for different powers.
• Use the Japanese four-digit scale, so 234909023 becomes 2.3億 and 2349090 becomes 234.9万. Preserve the existing formatting path for catalogs that do not opt in.
• Document the catalog entries and test real compiled translations, rounding, negative values, regional fallbacks, malformed profiles, locale isolation, and the translation update pipeline.

Validation: 873 ordinary tests passed on Python 3.13; Ruff 0.16.5, Black 26.5.1, Prettier 3.9.6, and mypy passed. The 15 benchmark tests were not run because their plugins were unavailable. A separate bounded timing check measured about 0.16 microseconds of additional overhead for English intword calls.

This uses new opt-in catalog entries and leaves existing unit message IDs in place, keeping the change separate from the placeholder migration in #403. The existing French plural-selection rule and the English decillion/googol gap are unchanged.