__update metamethod — intercept writ es to existing keys (patch for 5.5.0)

Hello,

I've been thinking about the gap between `__newindex` (new keys only) and 
the need to intercept updates to existing keys. The proxy-table workaround 
works but has notable overhead. I put together a small patch (~95 lines) 
for Lua 5.5.0 to explore what a native solution might look like — I'd love 
to hear if this direction has been considered or if there are concerns I'm 
missing.

The patch adds an `__update` metamethod that fires when assigning to an 
existing key (including deletion via nil), complementing `__newindex`. 
Same-value writes are skipped as a fast-path optimization.

```lua
local t = setmetatable({}, {
    __update = function(t, k, v)
        rawset(t, k, v)  -- or do validation, logging, etc.
    end
})
t.x = 1   -- fires (new key, no __newindex)
t.x = 2   -- fires (existing key, different value)
t.x = 2   -- no fire (same value, fast-path skip)
t.x = nil -- fires (existing key, deletion)
```

This eliminates the proxy-table pattern (`__newindex` + `__index` + 
`__pairs` on an empty table). Tables without `__update` have zero overhead 
— a single bit flag gates the fast path.

Full proposal, implementation details, and benchmarks:
https://github.com/Ne9roni/lua-update-metamethod

Feedback welcome!

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