docs(studio): SCIENCE-2 and SCIENCE-3 answered; ERA5 confirmed

[aliakherati]

Three decisions recorded (Ali, 2026-08-18)

With these, every science question gating Phases 1–4 is closed — what remains open is
SCIENCE-4's sedimentation half, SCIENCE-6 (ion-pair derivation), and SCIENCE-7 (upper bounds).

SCIENCE-2 — t = 0 is the moment a volume is defined

Engine exit and wake-vortex breakup are out of scope: the model starts from a user-specified
parcel and says nothing about how it came to be. No t0_definition field, no early-regime
parameterisation — the same reasoning as the heating/buoyancy decision, since a wake treatment needs
a different model.

The caveat changes character rather than disappearing: nucleation is strongly nonlinear in H₂SO₄,
so absolute particle numbers are conditional on the chosen initial concentration. Comparisons within
an ensemble sharing a V₀ convention are clean. CAVEATS.md states this as a scope boundary now, and
the stage-3 concentration-vs-volume panel stays — reframed as your result's sensitivity to a choice
the model does not make for you.

ASSUMPTION-5 (defaults follow the golden runs, not the spec's 10 m × 30 m) becomes permanent; it was
waiting on this.

SCIENCE-3 — pure sulfate, by assumption, revisitable

Recorded as ASSUMPTION-8 with the code location. The backgrounds were already seeded sulfate-only
(Mk[:, SRTSO4]), so this converts implicit behaviour into a dated decision with a single place to
start from when it is revisited. No meteoric material, no organics, no mixing-state question. The
per-dataset dry vs ambient diameter basis survives as a data-description note inside the
assumption record.

Climatology source — ERA5, alternatives tracked

Confirms BLOCKING-5's answer (Copernicus CDS, derived products committed with attribution). The new
issue #94 tracks other platforms — notably MLS for H₂O, since reanalysis stratospheric water
vapour is biased dry and H₂O is the field the sulfate microphysics is most sensitive to. Task 1.1
proceeds on ERA5.

Follow-through in code

• Stages 1 and 2 lose their blocked_on labels — no stage in the wizard is blocked any more.
• Every docstring and panel hint that described SCIENCE-2 as open now states the answer.
• Issues Studio · SCIENCE-1 — Climatology sampling convention (ANSWERED 2026-08-18: zonal-mean monthly) #53 / Studio · SCIENCE-2 — Definition of t = 0 (ANSWERED 2026-08-18: the moment a volume is defined; jet/vortex out of scope) #54 / Studio · SCIENCE-3 — Aerosol composition (ANSWERED 2026-08-18: pure sulfate, ASSUMPTION-8) #55 retitled ANSWERED, with the decisions quoted.

No schema fields changed; the config hash is untouched. 346 Python Tier-A, 55 vitest, both lanes
green locally (output read, not exit codes).

Stacked on #93 → #92 → #90. Merge bottom-up and I'll rebase anything GitHub flags.