fix(pricing): longest-match override keys, lenient band deserialize

CLAUDE_LONG_CONTEXT_PREFIX lists both "claude-sonnet-4" and its more
specific generations, and first-match resolution let the shorter row
shadow them — identical constants today, but an edit to one
generation's premium would silently never apply. Prefix overrides now
prefer the longest matching key (pinned by test). Also drops a
needless per-band clone in tier parsing and fixes the module doc's
stale claim that overrides apply at trim time.

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