CEG: add "parent age" operator

There's the `d` operator for trails which gives the parent projectile TTL. But TTL projectiles with a longer range have a higher TTL at start so it's difficult to make a generic effect (and it's generally inconvenient to work with values that count downwards and don't start from zero):

The request is to add a new operator that works like `d` but starts from 0 and counts upwards. Ideally, `the new operator + d` would be constant.

Start by locating the CEG implementation of the existing `d` trail operator and read how its value is calculated and exposed. Add the requested parent-age operator with a zero-based, upward-counting value, and verify that combining it with `d` remains constant as described.

Written by the indexing model from the issue text.