feature: add net.shared

[PyXiion]

Parent: #13

Add the network-synchronized reference built on the ref API.

Required API:

local state = net.shared("state", initial, {
  player = player,
  player_write = false,
})

state:get()
state:set(value)
state:update(function(old) return new end)
state:watch(function(new, old) end)
state:ready(function(state) end)
state:await()

Acceptance criteria:

• Server owns shared values.
• Client lookup returns a pending handle.
• Initial synchronization is separate from change watchers.
• Updates batch until tick end.
• Multiple updates in one tick are coalesced.
• Player-bound values are visible only to the target player.
• Read-only clients cannot mutate values.
• Writable clients can submit replacements when enabled.
• Serialization and size limits are enforced.
• Wire representation remains internal to Ignis.