v0.22.1: per-font weight boost, grow-to-fit template, config env var

The built-in template now grows the terminal to fill its stage, not just shrink
it. A mirror smaller than the viewport used to sit at its natural size in the
top-left; it now scales up to fit as well as down, so the terminal fills the
window at any size.

The renderer's weight boost is now a per-font setting. The kitty-parity
double-draw thickens anti-aliased midtones, which some fonts' own hinting
already handles — on those it reads as too heavy. Set weight_boost = false in
a [fonts."Name"] entry to turn it off for that family; omit it to keep the
boost, so every existing config renders exactly as before.

The config path now falls back to $SHELLGLASS_CONFIG when --config is
absent, so a shell profile can point at a config once instead of on every
invocation. An explicit --config still wins.