Add optional Frontier and BP-Trellis Python batch decoders

[ciaranra]

Frontier and BP-Trellis can now decode Python SampleBatch data and fused DemSampler shots with multiple workers. Their configuration factories and native workers live in the optional pecos-rslib-exp extension:

from pecos_rslib_exp import frontier, bp_trellis

result = batch.decode(dem_text, frontier(k=64), workers=4)

Both factories expose the underlying decoder configuration, including column ordering and BP-Trellis escalation. Standard pecos.decoders imports work without the experimental package; explicitly importing frontier or bp_trellis from that module lazily loads the optional extension.

The stable extension accepts a versioned Python decoder provider and retains shared batch execution, retries, scoring, and prediction handling. Each worker builds its own native decoder. Native construction and decoding release the GIL, with a Python call boundary per shot. Experimental specs currently support batch and fused sampling APIs, rather than stable DecoderSpec parsing or composition. The publishable pecos-decoders crate has no experimental dependencies.

Configuration is validated once: TrellisConfig::validate() and BpTrellisConfig::validate() own every rule that does not depend on a DEM, and the spec factories and the direct from_dem / from_factors constructors all call them, so an invalid option raises the same ValueError with the same message everywhere. TrellisOrdering now lives in pecos-trellis with a resolve(dem) method and is re-exported from pecos-bp-trellis and pecos-frontier. An exception raised while a provider builds its worker reaches the caller unchanged, with its original type and traceback.

The Guppy DEM workflow demonstrates both decoders and requires the optional extension, as its per-shot confidence section already did. The decoder guide documents configuration and the optional imports.

Validation:

• just pytest-ci-core: 2338 + 106 + 7475 + 113 passed.
• 94 Rust tests passed across pecos-trellis, pecos-bp-trellis, and pecos-frontier.
• The GIL-release tests time a single native build call and a single native decode call while a Python thread records its longest stall. Removing the GIL release from either call fails exactly the matching tests.
• A barrier-based test confirms that the provider bridge runs the requested number of workers concurrently.
• Worker scaling on a distance-5 circuit-level surface-code DEM (4000 shots, 1 to 16 workers): Frontier 10.9x, BP-Trellis 8.1x, against 7.8x for the built-in BP+OSD specification.
• Clippy with all targets and warnings denied on every crate that depends on the changed crates; rustfmt, pre-commit, and the Python lint recipe passed.

SampleBatch.decode(..., workers=N) now bounds N by the shot count before building its thread pool, matching the bound DemSampler.decode already applied, so an oversized request no longer spends its time spawning idle threads; workers_used reports the bounded count. Fixes #790.

Replaces #785, which closed when its head branch was renamed.