BUG: evaluate parachute triggers once per time node (#1086) - #1121

Conflicts, both from RocketPy-Team#1121 ("evaluate parachute triggers once per time
node"), which landed after this branch was cut:

- rocketpy/simulation/flight.py: RocketPy-Team#1121 moved the parachute loop out of
  __simulate into __check_and_handle_parachute_triggers, and in doing so
  already changed the trigger evaluation time from self.t to node.t. This
  branch had made the same self.t -> node.t change by hand, so that part of
  it is now redundant upstream. Resolved to develop's extracted helper; the
  only thing this branch still changes in flight.py is the one line setting
  parachute._eval_time.

- tests/unit/test_parachute_triggers.py: both sides appended a test to the
  end of the file. Kept both.

Two follow-ups while here:

- Parachute.__init__ now declares self._eval_time = None next to
  _trigger_falling_only and _trigger_needs_height, so the attribute is part
  of the object's state rather than something Flight conjures onto it. The
  trigger closure reads self._eval_time directly; the getattr default was
  unreachable anyway, since triggerfunc only exists once __init__ has run.

- docs/user/parachute_triggers.rst gained a "Fixed-time trigger" section.
  That page documents the "apogee" and numeric-altitude forms and tells
  readers a burnout delay needs a custom trigger, so leaving the new
  built-in form out of it was the one real gap. Includes the
  ("time", motor.burn_out_time + delay) composition and a note that firing
  at speed produces large loads.

Verified: tests/unit 2131 passed, 17 skipped; the new integration test and
the three parachute unit modules pass; ruff check and format clean; pylint
10.00/10. Checked in a real flight that ("time", 3.0) and ("time", 8.0)
deploy at exactly those times while still ascending (vz +268 and
+192 m/s), and that a plain "apogee" chute still fires at apogee.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>