Render signed PHIR registers as two's-complement bits instead of a sign prefix

[ciaranra]

Problem

When a HybridEngine + PHIR program produced a negative value in a signed
register (a sign bit that flipped), the returned bit string used Python's
sign-and-magnitude form — a leading "-" for negatives, "" for positives.
So a full-width signed register printed a "-" plus magnitude bits instead of
a fixed-width two's-complement string.

eng = HybridEngine(qsim="stabilizer")
results = eng.run(program=phir, shots=2)   # i32 register holding 0 - 1
print(results["out"][0])
# before: -00000000000000000000000000000001
# after:  11111111111111111111111111111111

Fix

Render the raw two's-complement bit pattern in the results() binary-string
path: mask the value to size bits and format that. The sign bit now shows up
as a "1"/"0" like every other bit — never a "-" prefix — and the width is
stable regardless of sign.

Applied consistently to both interpreters:

• phir_classical_interpreter.py (results()) — the default Python path
• classical_interpreter.rs (results()) — the Rust interpreter twin, kept in parity

Width stays tied to the declared register size. Negatives only occur when
size == the backing type width (e.g. i32 size-32, i64 size-64), so that
case already prints the full 32/64 two's-complement bits with the sign bit as
the MSB, while a narrow signed register (e.g. an i64 size-1 measurement bit)
still prints exactly one bit.

The general-purpose dtype __format__ in dtypes.rs is intentionally left
alone — it mimics Python int sign-magnitude semantics for direct
format(i32(-1), 'b') calls and other callers rely on it. This change only
touches the register result-string path.

Verification

• Reproduced the - bug end-to-end, then confirmed fixed across i8/i32/i64/u32,
  narrow and full-width registers.
• Python and Rust interpreters produce identical strings (parity).
• Added a regression test on each side.
• Updated test_phir_64_bit.py, which had been asserting the old
  sign-magnitude round-trip.
• Full local sweep: 717 Python passed, 86 Rust passed; lint + clippy clean.