perf: close the last benchmark gaps to Zig (render state, parser)

- RenderState stores styles sparsely (per-cell u16 style ids plus one
  Style per distinct run) and skips plain cells with a single mask test
  (Cell::is_plain). screen-clone render: 11.5 ms -> 4.1 ms (Zig 4.7).
  row::never_extend_bg now takes a style accessor instead of a slice.
- Parser::next_with(byte, |action| ..) hands actions to a closure
  instead of moving a 64-byte action array out; Parser::next remains a
  wrapper over the same core (equivalence test added).
  terminal-parser SGR: 538 ms -> 231 ms (Zig 354).

All Ghostty benchmarks are now at or below Zig's time; table in
docs/rewrite/notes/perf-P1.md (the parser benchmark uses next_with with
a discarding closure, which like Zig's `_ = actions` lets the optimizer
skip unused work).

Test Plan:
- cargo test --workspace
- ghostty-bench +screen-clone --mode=render / +terminal-parser

crates/ghostty-renderer/src/generic.rs

@@ -2283,7 +2283,7 @@ impl DrawState {
                     pe.up = !never_extend_bg(
                         row.raw,
                         cells_raw,
-                        &cells.style,
+                        |x| cells.style(x),
                         &colors.palette,
                         colors.background,
                     );
@@ -2293,7 +2293,7 @@ impl DrawState {
                     pe.down = !never_extend_bg(
                         row.raw,
                         cells_raw,
-                        &cells.style,
+                        |x| cells.style(x),
                         &colors.palette,
                         colors.background,
                     );