feat(pricing): keep models.dev context tiers and fast multipliers in the catalog

The catalog previously flattened models.dev to four flat rates, so
long-context tiers and fast/priority speed pricing were unpriceable
downstream. ModelPricing now carries the model's long-context band
(whole-request above-threshold rates + threshold from models.dev
cost.tiers, lowest context bound wins) and a fast-speed multiplier.

Neither vendors nor models.dev publish fast-tier pricing, and
models.dev's first-party Anthropic entries omit their long-context
premium (it only appears under gateway spellings the provider allowlist
drops), so both are hand-tracked override tables applied at trim time:
fast multipliers ported verbatim from ccusage's overrides (MIT), and
Anthropic's published 200K premium for the 1M-context sonnet/opus
models.

Cache rates become Option so unpublished rates stay distinguishable
from explicit zeros; cache_read_rate()/cache_write_rate() accessors
keep ccusage's models.dev-loader defaults (0.1x / 1.25x input). The
snapshot is regenerated with the new fields; old snapshot/cache JSON
still parses (missing fields default to None / 1.0).

Cost math is unchanged in this commit beyond the 1.25x cache-write
default for unpublished rates (cost-inert for Codex, which reports no
cache writes); consumers of the new tier fields land separately.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>