BUG: rocket with a late-starting thrust curve never leaves the rail (#411)

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Pull request type

• Code changes (bugfix, features)

Checklist

• Tests for the changes have been added (regression test fails without the fix, passes with it)
• Lint passes locally (ruff check, ruff format --check; pylint 10/10 on the touched file)
• Affected flight tests pass locally (tests/unit/simulation/test_flight.py, 56 passed)
• CHANGELOG.md updated

Current behavior

Fixes #411. A motor whose thrust curve starts at t > 0 (e.g. burn_time=(8, 20)) never lifts the rocket off the rail — the flight stays at (0, 0, 0) with out_of_rail_time == 0.

Root cause (as the issue originally guessed): the rail phase's only time nodes are [t=0, max_time]. With no thrust at t=0 the rocket is stationary, so LSODA — whose max_step defaults to inf — takes one huge step that jumps clean over the burn. The thrust is never sampled and the rocket never accelerates. A normal motor accelerates at t=0, forcing small steps, so it only bites for late-starting curves.

Minimal repro (same rocket and total impulse, only the start time differs):

[thrust starts t=0] out_of_rail_t=0.329  apogee=10329 m
[thrust starts t=8] out_of_rail_t=0.000  apogee=0 m   <- never leaves the pad

New behavior

__setup_phase_time_nodes now forces a solver time node at the motor's ignition and burn-out whenever burn_start_time > 0, so the burn is always sampled. With the fix the delayed case flies identically to the t=0 case, just shifted:

[thrust starts t=8] out_of_rail_t=8.329  apogee=10329 m

The change is guarded to burn_start_time > 0, so ordinary motors (which ignite at t=0) hit none of the new code and are byte-for-byte unaffected — confirmed by the unchanged t=0 result above and the existing flight tests staying green.

Breaking change

• No

Additional information

Adds test_flight_with_delayed_burn_leaves_rail in tests/integration/simulation/test_flight.py; it asserts a burn_time=(8, 20) rocket leaves the rail after ignition and reaches altitude. It fails on develop and passes with this change.

Closes #411