fix: apply `LEAN_STACK_SIZE_KB` to the libuv event loop thread (#16376)

This PR makes `LEAN_STACK_SIZE_KB` also apply to the libuv event loop
thread.

That thread is started during runtime initialization, before the
variable used to be read, so it always reserved the default 1 GB stack
and programs failed with `failed to create thread` under address-space
limits such as AWS Lambda. The variable is now read in
`initialize_thread`, which precedes `initialize_libuv` in
`lean_initialize_runtime_module`; this replaces the reads in
`lean_run_main` and `lean_main`. `--tstack` still overrides it for
threads created after option parsing.

Fixes #16365

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>