perf(input): drop buffheader_T #41353

Problem:
`buffheader_T` is a linked list (Vim's favorite data structure) with
complex bookkeeping, optimized for ancient hardware:

1. Memory topology: On old platforms (Amiga 512KB without MMU, MS-DOS
   64KB segments), large contiguous allocations were hazardous (no VM).
2. Never-move appends: `add_buff()` fills spare space or links a new
   block.
3. OOM "recovery": a failed block allocation only loses one append.

To avoid OOM, it prefers to allocate small, linked chunks, instead of
resizing one continguous slice. Each stuff/drain cycle costs
a malloc+free.

Solution:
Replace Vim's favorite data structure with Nvim's favorite data structure.

Nvim doesn't have granular handling of OOM (`xmalloc`), and hardware has
changed: caches favor contiguous memory, allocators handle
fragmentation. And these buffers are ~kb scale, so OOM is irrelevant on
any system that can run Nvim.

- Use a flat `StringBuilder` + `read`/`insert` offsets.
- Keeps capacity (does not shrink) until `free_buff`.
- "Steady state" allocates nothing: 1 fewer malloc+free per dot-repeat.

0 files changed, 0 insertions, 0 deletions