wasm: resolve fixed-array .len instead of aborting with "could not find field len"

[enghitalo]

Description

Reading .len on a fixed-size array aborts the wasm backend with an internal error:

fn main() {
	a := [8]int{}
	println(a.len)
}

wasm error: could not find field `len` on init

Root cause

The checker types array_fixed.len as int (checker.v) but emits no actual field for it. In the wasm backend, expr()'s ast.SelectorExpr arm tries get_var_from_expr (fails for .len), then falls through to field_offset → get_field_offset → ts.find_field('len'), which fails and calls w_error('could not find field len on init').

Fix

A fixed array's length is a compile-time constant — ast.ArrayFixed.size. Resolve .len directly in the SelectorExpr arm and push it as an i32 const, before the field-offset path.

ast.SelectorExpr {
    final := g.table.final_sym(node.expr_type)
    if node.field_name == 'len' && final.info is ast.ArrayFixed {
        g.func.i32_const(i32(final.info.size))
    } else if v := g.get_var_from_expr(node) {
        ...

After

$ v -b wasm run a.v
8

Tests

Extends vlib/v/gen/wasm/tests/arrays.vv (+ .out) with a fixed-array .len case (compiled and run via v -b wasm run, output compared). Full vlib/v/gen/wasm/tests/ suite passes; v fmt clean.

Checklist

• v test vlib/v/gen/wasm/tests/ passes
• v fmt -w applied