[mitchellh]

This focuses explicitly on the non-mmap allocations for a ghostty_terminal_new result. The result is that we lower this portion of the memory by almost half. The total benefit is smaller since 70% of a terminal is mmap'd allocations, but this still yields an absolute ~5KB savings on macOS on every new terminal (not just empty, but also with a normal prompt and so on).

Four changes to make it happy, broken down into individual commits. Nothing crazy:

• Page list nodes are pooled individually. The node pool was a std.heap.MemoryPool, which sits on an arena that preheats and grows 1.5x, so we paid for wasted space. Nodes now come from UntouchedPool (the same pool as page buffers) with a preheat of one, so the cost is exactlyone node (well, exactly one bucket element size in whatever allocator).
• Pin pool and tracked pin set are sized for two pins. Every screen tracks exactly a viewport pin and a cursor pin at creation, but we preheated eight pins and let the tracked pin map grow to 17 slots on the first insert via doubling.
• The kitty temp dir path is allocated only when set. The C wrapper embedded a 1 KiB max_path_bytes buffer that only embedders that set kitty_image_medium_temp_file ever wrote to.
• The default palette is shared instead of copied. DynamicPalette carried two full 1 KiB palettes, current and original, and original was almost always the built-in default. It is now a pointer to the shared built-in default, or to an allocator-owned copy when a custom default is set. This introduces a new OOM path but we gracefully handle it by either ignoring or resetting.

Memory measurements:

Per terminal	Before	After
phys_footprint delta, fresh	30,066 B	25,069 B
phys_footprint delta, styled prompt written	47,023 B	41,944 B
malloc zone bytes dirtied, fresh	12,698 B	7,782 B
malloc blocks live after terminal_new	11,904 B	6,688 B
malloc blocks live after the prompt	12,320 B	7,104 B

I ran ghostty-bench +terminal-stream on a 500 MB ascii corpus, main vs this branch interleaved, and there is no noticeable change.

AI usage: Fable did validation of the work, I did the implementations, commit messages, and PR notes.