Refactor Gemini simulator tasks around backend objects

[Krastanov-agent]

Draft PR for #789.

cc @Krastanov

Summary

• add a minimal simulator backend layer with Tsim, CliffT, and PyQrack backends
• move logical and physical simulator config from task() into simulator constructors
• route logical and physical task run() through backend sampling with consistent signatures
• add GeminiPhysicalSimulator while keeping PhysicalSimulator as a compatibility alias
• update demos for the new constructor-owned config/backend API

Breaking changes

• GeminiLogicalSimulator.task() now takes only the application kernel; m2dets and m2obs are simulator constructor arguments
• physical simulator task() now takes only the physical kernel; placement and annotation config are simulator constructor arguments
• GeminiLogicalSimulator(backend="clifft", seed=...) is replaced by GeminiLogicalSimulator(backend=CliffTSimulatorBackend(seed=...))

Verification

• uv sync --locked --dev --all-extras --index-strategy=unsafe-best-match
• uv run --locked pytest python/tests/ -q -> 1520 passed, 9 skipped
• uv run --locked ruff check ... on touched files
• uv run --locked black --check --target-version py312 ... on touched files
• uv run --locked pyright python/bloqade/gemini/device python/bloqade/gemini/decoding/experiments.py

Notes

• PyQrack records raw measurement samples and uses Tsim for detector error model generation so public result containers keep a DEM when available.
• Tsim-specific compatibility properties (tsim_circuit, samplers) remain available for Tsim-compatible backends because decoding utilities still use them.

[codecov]

Codecov Report

❌ Patch coverage is 72.29437% with 64 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
python/bloqade/gemini/device/simulator_backend.py	74.12%	37 Missing ⚠️
python/bloqade/gemini/device/physical_simulator.py	54.34%	21 Missing ⚠️
python/bloqade/gemini/device/simulator.py	85.00%	6 Missing ⚠️

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