[lldb][debugserver] Expedite the stopped frame's stack memory in jThreadsInfo (#212706)

Add `ReadFrameZeroStackMemory`, which expedites the innermost frame's stack
memory so a variables view on a stop is served from lldb's memory cache.  When
frame 0's `$fp` looks usable, two windows are expedited:

  * `[$fp + 2*ptr_size, $fp + 2*ptr_size + k_expedite_stack_arg_size)` for stack-passed
     parameters, starting above the saved `{fp, lr}` pair the backchain already covers.
  * `[$fp - below, $fp)`, `below = min($fp - $sp, k_expedite_stack_window - k_expedite_stack_arg_size)`,
     for locals and spilled register arguments.  A small frame gets all of `[$sp, $fp)`; a large one keeps the part nearest `$fp`, so the cost stays bounded.

If `$fp` fails validation (frameless leaf, or `$fp` used as a scratch GPR),
a single `[$sp, $sp + k_expedite_stack_window)` window is expedited instead.

Each window is a separate chunk, because lldb's L1 cache only serves reads
fully contained in one expedited chunk.  Only the thread that stopped gets
these windows, so the stop reply does not grow with thread count.

`GetJSONThreadsInfo` now builds the `"memory"` array from both sources and
emits it whenever either produced an entry.  Add `JSONGenerator::Array::empty`
for that check.

GitOrigin-RevId: b631e0cbd1c9e33f29a199b3028026ded89167bd