fix(beam): support module-level mutable variables via process dictionary

[dbrattli]

Module-level let mutable a = 10 was broken: the declaration compiled to a constant-returning function a() -> 10, but assignments (a <- 20) created an unrelated local variable, and reads still called a() (returning the stale initial value).

Route module-level mutables through the process dictionary

The same mechanism already used for local mutable let bindings:

• MemberDeclaration for a mutable value (no args, IsMutable) now emits a main/0 that initialises via put(name_atom, value). The existing multiple-main/0 merge logic combines it with the ActionDeclaration body, so the initialisation always runs first.
• IdentExpr for a mutable ident not in ctx.MutableVars/LocalVars now emits get(name_atom) instead of calling the (now-absent) function.
• Set(IdentExpr, ValueSet, ...) for a mutable ident not in ctx.MutableVars now emits put(name_atom, value) instead of a local match.

With this change let mutable a = 10; a <- 20; printfn "A=%A" a correctly prints A=20.

Snapshot immutable values that read a module-level mutable

An immutable module value that reads a module-level mutable (e.g. let c = topA) was compiled as a lazy 0-arity function that re-read the live process-dictionary value, so it observed later reassignments instead of the value at binding time. F# evaluates such bindings once at module init — before any later do/reassignment — so they must be snapshotted. They now emit a main/0 init (put(name, value), in declaration order) plus an accessor that reads the snapshot. Only values reading a free mutable are affected; self-contained bodies (local mutables) stay lazy to avoid a spurious dependency on main/0.

Run module initialisation before tests

The Erlang test runner called test_*/0 functions directly and never ran main/0, so module-level initialisation (mutable inits and do actions) never executed and reads returned undefined. The runner now invokes a module's main/0 before its tests, mirroring .NET module initialisation (which runs before any module code).

Tests

Enables the previously-skipped Setting a top-level value doesn't alter values at same level test and adds dedicated read/write, multiple-assignment and cross-function module-level mutable tests. Full beam suite: 2457 passed, 0 failed.

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This was previously pushed directly to main (488f4cd) and has been moved to a PR.

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