fix(#292) populate alpha_i/beta_out/psi/omega output extras after forward()

[prjemian]

• closes psic: fixed alpha_i modes #292

The fixed_alpha_i_, fixed_beta_out_, alpha_eq_beta_, fixed_psi_,
and fixed_omega_* modes declared placeholder slots in mode.extras
for derived angles that the forward solver did not constrain
directly. Before this PR those slots remained at their YAML default
of None even after a successful forward() call — an advertised
output contract that the solver did not honor.

Adds _populate_output_extras() to compute_forward(), called
after _validate_solutions(). For each output-slot key declared in
the active mode's extras (alpha_i, beta_out, psi, omega),
it writes a list of per-solution float values computed via the
matching helper in ad_hoc_diffractometer.reference. Empty
solution lists reset the slot to []; a raising reference helper
leaves the slot as None and logs the cause at DEBUG.

Adds the missing Extras (output) rows to the affected psic modes
in docs/source/geometries/psic.md — sixc.md, zaxis.md, and
s2d2.md already had them. Adds the Fixed bullet to
CHANGES.md.

New regression test tests/test_regression_issue_292.py (12 cases)
covers: B3 surface mode populates alpha_i + beta_out against an
independent recomputation; fixed_omega_* populates omega;
fixed_psi populates psi; empty solutions reset to []; second
forward() overwrites the prior contents; modes without output
slots are untouched; soft-fail leaves the slot as None and logs at
DEBUG.

Verification on wow.xray.aps.anl.gov (Python 3.14.4, NumPy 2.4):

• pytest: 2592 passed, 2 skipped, 3 deselected — 100.00 % coverage
• pre-commit run --all-files: clean
• make -C docs clean html: clean build, no warnings, no RST-leak
  patterns
• US-English spelling check on changed files: clean

Contributed by: OpenCode (argo/claudeopus47)