Support top-level bindings in the self-host pipeline

[jasisz]

A binding written outside any fn is a module-level value. The host VM evaluates every top-level statement in source order before calling main and exposes the results to all functions. The self-hosted interpreter evaluated those statements into a throwaway environment and then called main with an empty one, so this program worked on the VM and failed under aver run --self-host with undefined variable: greeting:

greeting = "hello"

fn main() -> Unit
    ! [Console.print]
    Console.print(greeting)

What changed

• FnStore carries a globals map next to the function table, with withGlobals / lookupGlobal accessors.
• evalTopLevelStmts threads the environment through the top-level statement list and returns it, re-attaching it to the store for each statement, so a binding whose value comes from a function call can read the bindings above it.
• The unresolved-name fallback consults top-level bindings before function references, matching the host, where a binding sharing a function's name wins when the name is read as a value.

Semantics were derived from the host VM with probe programs first: source order, visible to functions defined before the binding, forward references and function-local rebinding rejected by the typechecker on both paths, parameters and match-arm bindings shadow normally.

src/self_host was regenerated through the release regeneration sequence; the diff is confined to the three evaluator modules. The regeneration gate passes (AVER_SELF_HOST_REGEN=1 cargo test --features runtime --test rust_self_host_regen -- --ignored, corpus parity 3/3). No unrelated drift showed up in the regenerated tree.

Tests

tests/cross_backend_stress.rs gains VM/self-host parity pairs covering a binding used in main, a binding reading another binding, a binding produced by a function call, shadowing by a parameter and by a match-arm binding, and a binding sharing a function's name.

Known gaps, unchanged

• The wasm-gc backend traps on any program with a top-level binding. Pre-existing and untouched here, so no wasm-gc test is added.
• The self-host HttpServer callback bridge builds its own function store without globals, so a handler reading a top-level binding still fails. Previously the whole program failed, so this is strictly narrower.

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