Suppress spurious -Wcast-align in the arena allocator

[brtnfld]

cell_realloc()'s header-before-payload pattern and its call sites that cast the returned char* back to a wider-aligned pointer type trigger -Wcast-align under -Werror. The casts are safe: p is always either REALLOC()'s own return value, or p - sizeof(cell_t) recovers that exact same, maximally-aligned pointer -- the compiler just can't prove it from byte-level pointer arithmetic on a char*. Route each cast through an intermediate (void *), the standard idiom for a deliberate alignment-increasing cast. No behavioral change.

[cktan]

can't repro

[brtnfld]

Re "can't repro": the warning only shows up if -Wcast-align is explicitly enabled — it's not part of -Wall/-Wextra, and the project's default src/Makefile CFLAGS (-std=c17 -Wmissing-declarations -Wall -Wextra -MMD) don't turn it on, so a plain make won't trigger it.

Steps to reproduce (against main before this PR):

git checkout main -- src/tomlc17.c
cc -std=c17 -Wall -Wextra -Wcast-align -Werror -c src/tomlc17.c -o /tmp/t.o

That fails with 8 -Wcast-align errors, e.g.:

src/tomlc17.c:224:16: error: cast from 'char *' to 'cell_t *' (aka 'struct cell_t *') increases required alignment from 1 to 4 [-Werror,-Wcast-align]
  cell_t *cp = (cell_t *)(p - sizeof(cell_t));

Building the same command against this branch's src/tomlc17.c compiles clean.

I also rebased the branch onto the latest main (your recent "cleanup" commit had reformatted a couple of the same lines this PR touches, which was causing the merge conflict) and re-verified with the -Wcast-align -Werror build plus make test (all suites pass except stdtest, which needs go/toml-test not installed in my sandbox — unrelated to this change). PR should be mergeable now.

[cktan]

i don't see it on ubuntu...

[cktan]%
[cktan]% cc -std=c17 -Wall -Wextra -Wcast-align -Werror -c src/tomlc17.c -o /tmp/t.o
[cktan]%