Require cudaq >= 0.16.0

[wsttiger]

Description

CUDA-Q 0.16.0 is released; this makes it the minimum supported
version. Four coordinated changes:

• pyproject.toml: the wheel now declares cudaq >= 0.16.0, < 0.17.
• CI pip lanes install the same range (kept in lockstep by design —
  PRs test exactly the claim the wheel ships).
• The Custom-mode wheel version placeholder moves 0.15.99 → 0.16.99 to
  stay above the released version line.

Validation

• Full suite run against the released cudaq==0.16.0 wheel ahead of
  this PR: 472 passed / 3 skipped on the primitives line (a superset
  of every test on main). v0.1.0-era functionality requires no code
  changes on 0.16.
• The one 0.16-sensitive construction (1-bit-address QROM lookups in
  the unary-iteration feature) ships with its own protection — the
  dispatch-pruning commit already on Add unary-iteration and QROM primitives #40 — so this PR and Add unary-iteration and QROM primitives #40 can
  merge in either order. Open PRs adopt the new range only when they
  rebase, at which point they inherit that commit too.
• This PR's own CI validates the switch: the pip lanes install 0.16.0
  and run the full suite against it.
• The .cudaq_version pin bump is deliberately split into a follow-up
  PR: its first firing of the from-source lanes surfaced a
  build-layout change in the 0.16 source tree (built install no longer
  exposes the python package on the expected prefix path) — the lanes
  doing exactly the job Test PRs against released cudaq; from-source becomes a manual probe #42's review designed them for. The pin moves
  once the build plumbing adapts.

Notes

• 0.16.0 ships without the fix for cuda-quantum#5280 (verified in the
  release tag); with the pruning commit merged, nothing in this
  library triggers it.
• 0.16 lifts the control-variant restriction on kernels calling
  kernels; follow-up modernization (composed select_swap, composition
  contract) is staged on features/primitives_016_refactor and lands
  separately after this.

[anjbur]

We'll also need to bump the 0.15.99 version used in get-cudaq-wheels/action.yaml up to 0.16.99. Otherwise, the CI will break for Custom wheel builds.