Fix #56: derive hasMeasurement() from the gates instead of a flag

[corgab]

Summary

CircuitBuilder kept a private $hasMeasurement flag that push() had to set, while gateCount() and depth() derived the same fact by scanning $gates. The flag is gone: the question is answered from the gates, the way its siblings already do.

Plan

1. Remove the field and its assignment in push().
2. Add a public hasMeasurement(): bool that scans $gates for a measurement (a plain loop, since array_any() is PHP 8.4 and the package supports 8.3), and have validate() call it.
3. Pin the behaviour with tests, including the case the flag would have got wrong if append() ever changed: a measurement dropped from an appended fragment does not count.

Changes

• src/Circuit/CircuitBuilder.php: $hasMeasurement removed; hasMeasurement() added next to validate(), with a docblock explaining why it is derived; push() only validates and appends.
• README.md: hasMeasurement() listed with gateCount() and depth() in the inspection example.
• tests/Unit/Circuit/CircuitBuilderTest.php: false before measure(), true after; a fragment's measurement does not leak in through append(), and run() still refuses such a circuit.

Tests

• vendor/bin/pint --test passes.
• vendor/bin/pest --compact: 793 tests, 1 skipped (791 before: +2).

Closes #56

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Generated by Claude Code

[corgab]

Chiusa in vista della migrazione all'architettura 100% PHP. Mantenere il flag O(1) è la scelta migliore per le performance con circuiti nativi di grandi dimensioni.