gen_perceptron.t27: the reference passes a weight update Zig never analyzes (assignment through a by-value parameter) #7741

Part of #6063 (t27b coverage). Found while landing #7740.

specs/port/trinity/src/tri/gen_perceptron.t27 passes the reference (t27c gen + zig test, 5 of 5) only because Zig never analyzes the body of its weight update.

fn Perceptron_train(perceptron: Perceptron, inputs: []const f64, target: i32) -> void {
    ...
    for i in 0..inputs.len {
        if (i < perceptron.weights.len) {
            perceptron.weights[i] += perceptron.learning_rate * diff * inputs[i];
        }
    }
}

• perceptron is a by-value parameter, so the assignment writes through a constant. Zig refuses that ("cannot assign to constant").
• weights is [0]f64, so the condition is i < 0 on a usize. Zig decides it at compile time from the type's range, and does not analyze the arm.
• Measured on the t27c lab with zig 0.16.0: the same generated Zig with [1]f64 instead fails to compile at that line, error: cannot assign to constant.

The only test that reaches the fn, perceptron_training_update, asserts true. So the reference passes a weight update that can never run and would not compile if it could.

After #7740 t27b gets past const diff: f64 = target - guess; and refuses the assignment by name: StmtAssign (assignment through a constant) at line 38. That refusal is right. The ledger records this spec as reference-bug.

The spec fix is the port's, not t27b's: make the update a var copy or a *Perceptron parameter, and give it a test that reads a weight.

Boundary

• specs/port/trinity/src/tri/gen_perceptron.t27

User Scenarios

• Given the gen_perceptron.t27 spec defines Perceptron_train with a by-value perceptron parameter and zero-length weights, when t27b compiles the spec, then the assignment to perceptron.weights[i] is rejected as an assignment through a constant.
• Given the spec is fixed to use a var copy or pointer parameter for the perceptron and gives weights a non-zero length, when the generated Zig code is compiled and tested, then the weight update executes and a test observes the changed weight.

Requirements

• FR-001: The Perceptron_train function MUST accept the perceptron by pointer (*Perceptron) or operate on a var copy so that weight updates are not assignments through a constant.
• FR-002: The weights field MUST have a non-zero length (e.g., [2]f64) so the loop condition i < perceptron.weights.len is not trivially false and the loop body is analyzed.
• FR-003: The spec MUST include a test that calls Perceptron_train and asserts that a weight value has changed after training.

Success Criteria

• t27c specs/port/trinity/src/tri/gen_perceptron.t27 compiles without errors and exits 0
• zig test zig-out/bin/gen_perceptron_test.zig (or equivalent generated test binary) runs and exits 0 with at least 1 test that reads a weight after training
• The spec ledger no longer records gen_perceptron.t27 as reference-bug