Fix buffers being copied by reference in deep copies

[tktj12]

Problem

DeepCopyTable() and ReconcileTable() copy buffers by reference.
So the module-level template itself gets mutated.

Since JSONAcceptable (line ~970) explicitly lists buffer as a valid
profile data type, and the template is documented as a "hard-copy"
(line ~44), buffers should be hard-copied

Repro

local TEMPLATE = { Bits = buffer.create(4) }
local Store = ProfileStore.New("BufferRepro", TEMPLATE)

local a = Store.Mock:StartSessionAsync("playerA")
local b = Store.Mock:StartSessionAsync("playerB")

print(a.Data.Bits == b.Data.Bits)        --> true  (expected false)

buffer.writeu32(a.Data.Bits, 0, 123)
print(buffer.readu32(b.Data.Bits, 0))    --> 123   (expected 0)
print(buffer.readu32(TEMPLATE.Bits, 0))  --> 123   -- template is mutated

The template corruption persists for the whole server session.

Affected call sites

• MissingProfileHandle — every new profile shares the template's buffer
• Profile.New — LastSavedData aliases the live buffer
• MockUpdateAsync — mock store aliases live buffers, so Studio and production behave differently
• ReconcileTable — same issue when filling in a missing buffer key

Fix

Adds a CopyBuffer helper and a buffer branch to both copy functions.

Note: the trailing-newline change at EOF was added automatically by the
GitHub web editor and is not part of the fix.