class PmrStackArena

A stack-allocated PMR monotonic arena of `Size` bytes aligned to `Alignment`.

Because Fuchsia builds with `-ftrivial-auto-var-init=pattern`, a raw stack `std::array` or byte

array used to back a `std::pmr::monotonic_buffer_resource` would be `memset(0xAA)`-filled on

every function entry. `PmrStackArena` is annotated with `__declspec(no_init_all)` (enabled by

`-fdeclspec` in `//src/ui/scenic/lib/utils:pmr_stack_arena_config`), which instructs Clang to

skip automatic pattern-initialization for local variables of this class while still running the

constructor to initialize `resource_`.

`PmrStackArena` must be declared directly as a local stack variable (not on the heap, and not as

a member of an unannotated struct or class, because Clang checks `__declspec(no_init_all)` only

on the declared variable's type). `upstream` must be non-null and outlive the arena, and the

arena must be declared before (and outlive) all containers using `resource()`. `resource()` is

not thread-safe.

The raw backing storage `buffer_` is kept private so callers can only access it by allocating and

constructing objects through the `std::pmr::monotonic_buffer_resource`.