Fix #48: reject non-integer measurement targets with InvalidCircuitException

[corgab]

Summary

Gate::measure(int|array|null $targets) stored any array verbatim, so measure(['a']) passed the parameter type and failed later in CircuitBuilder::validateTargets(string $gate, int ...$qubits) as a raw TypeError, unlike every other circuit-shape mistake, which throws InvalidCircuitException. Worse, Gate::fromArray() decoded serialized values with (int): a corrupted queued definition carrying targets: ["a"] was silently rebuilt as a measurement of qubit 0, and the same cast turned target: "a" on an h gate into qubit 0 too.

Plan

Validate every qubit index once, in Gate, for both entry points: the fluent measure() call and the fromArray() decoding path, for measurements and for every other gate's qubit keys.

Changes

• Gate::integerIndex(string $gate, mixed $value): int (private): the value must be an int, otherwise InvalidCircuitException::invalidQubitIndex(GATE, $value). integerIndices() applies it to a list and reindexes it.
• Gate::measure() uses it for the array form. Gate::fromArray() uses it for every qubit key instead of (int), and decodeMeasureTargets() returns null only for a null targets, rejects a scalar (previously read as "measure all") and validates the elements; the empty-array rule stays in measure(), which every path reaches.
• InvalidCircuitException::invalidQubitIndex(string $gate, mixed $value): names the gate and the offending value (Gate H expects integer qubit indices, got 'a').
• README circuit section: one sentence on integer indices and on what happens to a corrupted queued definition.

No config change, no new dependencies, no change for valid integer indices (a keyed measure() array such as [2 => 1, 5 => 0] is now reindexed to [1, 0], which is what the Python side already expected; a keyed array previously serialized as a JSON object).

Tests

• GateTest: dataset of invalid measure() targets (string, numeric string, float, mixed with a valid index, nested array); explicit targets are reindexed; fromArray() dataset covering a string measure target, a scalar targets, a string h target, a numeric-string h target and a float cnot control, all throwing with the value in the message.
• CircuitBuilderTest: ->measure(['a']) throws InvalidCircuitException, not a TypeError.
• The datasets fail on main (TypeError, silent qubit 0, or silent measure-all).

Local: pint passed, pest 804 tests passed.

Closes #48