vm: 5.3-only loop-built-closure equality caching (closure.lua)

[ianm199]

On 5.3, closures built in a loop with identical upvalues compare == due to closure caching (closure.lua:48); lua-rs creates distinct closures. 5.4/5.5 match already. VM/upvalue-capture change, 5.3-gated. Also blocks goto.lua:180 on 5.3 (the same upvalue-id sharing across a goto-driven loop). Re-entry notes: specs/followup/SHARED_CORE_REPORT.md.

[ianm199]

Triage (2026-06-01): verified STILL-OPEN

Reproduced against current main:

local up=42; local t={}
for i=1,3 do t[i]=function() return up end end
print(t[1]==t[2])  -- 5.3: true (cached)   5.4+: false (no cache)

• LUA_RS_VERSION=5.3 → false (wrong; ref lua5.3.6 → true)
• LUA_RS_VERSION=5.4 → false (correct; matches lua5.4.7)

Seam is closure construction (state.rs::push_closure, dispatched from OP_CLOSURE in vm.rs); needs a per-proto LClosure cache keyed on the upvalue set, 5.3-gated (5.4 removed the cache). Medium-sized, localized. Labeled bug / 5.3 / priority: medium.