Re-evaluate the visualized variable's address on refresh

[tiresiasfromthebai]

Problem

The Array and Matrix visualizers evaluate the variable's address only
when the name is typed. Every refresh — the manual button as well as the
auto-refresh at each stop (#496/#498) — then re-reads memory at that
stored address, and never re-evaluates the name. So whenever the
variable's address changes, the plot silently shows stale (or wrong)
memory, and no amount of refreshing heals it:

• Enter &pos[0] for a std::vector before it is constructed (e.g.
  before Run, or at an early breakpoint): the address evaluates to 0x0
  and is trapped forever — every subsequent stop re-reads address 0 and
  pops "Unable to read memory", even though the vector now exists.
• A vector that reallocates (push_back, resize) between two stops: the
  visualizer keeps plotting the old buffer — plausible-looking but
  dead data, with no error at all.
• Same story after a re-Run of the program (fresh stack/heap addresses).

Fix

Split the refresh in two steps. The refresh handlers now re-evaluate the
variable name, and the ^done answer reads the memory at the fresh
address (new readaMemory()/readbMemory()/readMemory() holding the
previous read logic). Since every refresh path funnels through the same
handlers — the manual Refresh button, the auto-refresh at each stop, and
the length/rows/columns/offset/stride changes — they all pick up the
fresh address now. A nice side effect: re-typing the name while
stopped refreshes the plot immediately, instead of only updating the
address field.

The read step is skipped for a null address or an empty length: a
not-yet-constructed variable now shows its 0x0 address quietly and the
plot simply appears at the first stop where the variable exists, instead
of raising a memory error at every interaction.

Testing

Qt 6.4.2, Linux Mint. With a 4000-point std::vector phase portrait
(scatter, Auto checked):

1. Enter &pos[0]/&vit[0] at an early breakpoint (vectors not yet
   constructed), address shows 0x0 → Continue to the filling
   breakpoint, or simply hit Refresh: the address updates and the plot
   appears (previously: "Unable to read memory" on every stop and every
   refresh, forever).
2. Normal flow (names entered at a healthy breakpoint): stepping and
   Continue refresh exactly as before.
3. Matrix Visualizer: same two scenarios on an Eigen matrix.
4. Line/spline/scatter modes unaffected.