Tab-to-complete cannot join mid-token ("gi" → "git status" is rejected)

## Problem

The scorer only accepts a candidate that continues the typed text at a word boundary. A prefix that ends mid-word — `gi` when the corpus holds `git status` — scores −8 and is rejected, so the most common shell case gets no suggestion.

## What it needs

A P(token | prefix) estimate over the vocabulary, so a partial word can be completed before the rest of the line is scored. The corpus is on disk (Phase 2, #31) and the scorer is pure code with nothing attached (#30), so this is a self-contained change with fixtures to measure against.

Report before/after on the 29-fixture runner and the 1,090-case catalogue.