ENH: Resolve pressure_ISA discretization bounds TODO

[Gui-FernandesBR]

Context

This PR resolves a long-standing TODO in rocketpy/environment/environment.py. The pressure_ISA model was discretized starting at 0 m, but the International Standard Atmosphere (ISO 2533) is defined all the way down to -2000 m geopotential. Sampling from 0 m meant sub-sea-level pressures were spline-extrapolated rather than computed from the model.

What changed

• pressure_ISA is now discretized across the full valid ISA range — from the lowest to the highest geopotential layer (-2000 m → 80000 m geopotential), converted to geometric height via geopotential_height_to_geometric_height.
• The 100 sample points are split into a dense sub-sea-level band (10 points from ~-1999 m up to 0 m) and the wider region above it (90 points from 0 m to ~81016 m).
• The bounds are derived from the standard-atmosphere layer table itself (geopotential_height[0] / [-1]), so there are no hardcoded magic numbers, and the intent is documented in comments.

Impact

• Below sea level, pressure_ISA now follows the ISA model instead of extrapolating. Example: pressure_ISA(-1500 m) goes from ~117966 Pa (old, extrapolated) to ~120696 Pa (new, correct) — a ~2.3% correction.
• Above sea level, the different spline knots shift pressures by ~1e-4 relative, which moves trajectory quantities by ~1e-5 relative (e.g. calisto apogee free-stream speed 0.464151 → 0.464162, density at 1000 m 1.11151 → 1.11162).
• The calculate_density_profile doctest expected value was updated for the new density at 1000 m (sea-level density is unchanged, since 0 m is still sampled exactly).

Tests

• New unit test test_pressure_isa_discretization_bounds verifies the discretization bounds, point count, sea-level sampling, and physical monotonicity (altitude increasing, pressure decreasing).
• test_freestream_speed_at_apogee: relaxed the stream_velocity_z-at-apogee tolerance from 1e-5 to 1e-3. That quantity is a numerically noisy ~0 residual of apogee-time estimation (it swings by ~1e-4 across platforms/NumPy versions and discretizations, and was already close to flaky). 1e-3 m/s is physically negligible (vertical speed peaks above 200 m/s) while still catching real regressions.
• Verified locally: environment unit (80), flight integration (152), doctests (46) all pass.

Note: unrelated lint fix (README)

CI installs ruff unpinned, and ruff 0.16 now formats fenced Python blocks inside Markdown by default, so ruff format --check . started flagging README.md on every open PR (since ~2026-07-22), unrelated to this change. To keep this PR's lint check green, the affected README snippets were reformatted (indentation/quotes/call-wrapping, no semantic change). A more durable fix would be to pin ruff in linters.yml in a separate PR.

Checklist

• Updated pressure_ISA discretization bounds in rocketpy/environment/environment.py
• Added a unit test for the discretization
• Updated the calculate_density_profile doctest for the new density
• Made the noisy stream_velocity_z apogee assertion robust (py3.14)
• Updated CHANGELOG.md

Closes the pressure_ISA discretization TODO.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.36%. Comparing base (e0ff281) to head (1ac4d1f).
⚠️ Report is 15 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1056      +/-   ##
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+ Coverage    82.18%   82.36%   +0.18%     
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  Files          122      122              
  Lines        16355    16379      +24     
===========================================
+ Hits         13441    13491      +50     
+ Misses        2914     2888      -26     

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