Handle empty and tiny localization inputs without crashing

[kalidke]

Summary

frameconnect and its building blocks crashed on a BasicSMLD with 0 (or untyped/abstract) emitters: internal reductions (counts/mean/extrema) ran over empty collections, producing MethodError: zero(Type{Any}) from reduce_empty(+, Type{Any}) and "reducing over an empty collection". This surfaced from the SMLMAnalysis pipeline on near-empty dSTORM cells (some cells yield ~0 localizations) — the pipeline should degrade gracefully, not crash.

Changes

isempty guards at the empty-input sites so 0-localization data returns a passthrough/empty result instead of crashing:

• frameconnect — early-return the (empty) input + a zeroed FrameConnectInfo before the pipeline. When calibration is configured, it still emits a graceful fallback CalibrationResult (calibration_applied=false) so downstream code reading info.calibration.* sees a uniform shape for 0 vs 1–2 localizations.
• precluster — return the input unchanged for 0 emitters (guards counts on datasetnum/framenum and first(emitters)).
• combinelocalizations — return the input unchanged for 0 emitters.
• calibration.jl — no change needed; analyze_calibration already falls back at n_pairs < 100 before any reduction, and the empty branch reuses it.

Tests

New test/test_empty_input.jl covers 0/1/2 localizations × (default + calibration-enabled) × (concrete + abstract Emitter2DFit eltype), plus the combinelocalizations/precluster building blocks.

Full suite: 224/224 pass. 0-emitter BasicSMLD now returns (empty_smld, info) with n_combined=0; 1/2/9-loc inputs return cleanly; calibration-enabled empty/tiny → calibration_applied=false.

Notes

• Patch bump 0.3.2 → 0.3.3. Draft / branch-only — not for merge or registration in this state.
• Out of scope (separate, pre-existing): two localizations at exactly identical float positions → zero-area cluster → Inf density → Optim/LAP spins. Does not occur with real sub-pixel data; can be a follow-up if a real cell ever triggers it.

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