256 nested with panics: scope_depth is a u8

[wyattgill9]

$ fix eval --expr "$(python3 -c 'print("with { a = 1; }; "*255 + "a")')"
1

$ fix eval --expr "$(python3 -c 'print("with { a = 1; }; "*256 + "a")')"
thread 16530570 panic: integer overflow

scope_depth is a u8 (src/expr/compiler/context.zig:117), and beginScope increments it unguarded.

The panic is the good case. In ReleaseFast the increment wraps to 0, and endScope (src/expr/compiler/scope.zig:20) then pops against a bogus depth:

pub fn endScope(self: *Compiler) void {
    self.scope_depth -= 1;
    while (self.locals.items.len > 0) {
        const local = self.locals.items[self.locals.items.len - 1];
        if (local.depth <= self.scope_depth) break;
        _ = self.locals.pop();
    }
}

With a wrapped scope_depth the wrong locals are popped and the result is a silent scoping bug rather than a crash.

declareLocal already handles its own limit properly with error.TooManyLocals; the same treatment here would be consistent.

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fix 0.3.0 at 1f1a99cf, built -Doptimize=ReleaseSafe with Zig 0.16.0, darwin/aarch64. Reference implementation is Nix 2.34.8.