RFCs: Event-driven noise and leakage-aware channels

[qciaran]

Summary

This specification-only PR now contains two linked RFCs:

1. Event-Driven Noise Modeling defines the execution, placement, runtime, and compatibility architecture.
2. Leakage-Aware Stochastic Channels defines population-transition and Pauli-plus-leakage channel semantics.

The split allows reviewers to assess the channel mathematics independently from the broader runtime migration while retaining one coherent design.

Event-driven noise RFC

• represents gates, preparation, measurement, reset, idle, batches, circuits, and generic triggers as typed event anchors
• keeps phase, targets, timing, measurement outcomes, idle duration, and generation origin typed rather than hiding built-in data in opaque metadata
• separates stable serialized TriggerId values from process-local compiled IDs
• compiles broad and specific selectors into one deterministic total order using registration indices
• preserves runtime batch boundaries for crosstalk, timing, and simultaneous-operation semantics
• defines execute, suppress, and replace gate dispositions with conservative conflict rejection
• prevents ordinary gate-noise recursion on noise- and replacement-generated operations
• specifies a thin GeneralNoiseModel compatibility facade with no independent sampling logic
• requires exact seeded/RNG-stream parity for the compatibility facade; distributional parity applies to independently composed models
• proposes PECOS trigger commands and a provisional coordinated Selene Custom trigger transport
• requires a separate review before changing the production general-noise default

Leakage-aware channel RFC

• separates conditional population-transition channels from Pauli-plus-leakage channels
• defines overall channel probability versus per-source conditional normalization
• defines one- and two-qubit channels, independent versus shared outer coins, and tensor/composition algebra
• makes * an identity wire that never resolves or collapses the untouched qubit
• preserves simultaneous *L and L* rules with documented shared-coin semantics
• establishes classically tracked leakage as PECOS's backend-independent default
• makes leakage-to-depolarizing conversion an explicit numeric, reported approximation
• keeps compatibility p1/p2 convenience methods while compiling them into event rules

Context and conformance

Draft PR #518 remains an exploratory transition-channel implementation prototype. The experimental pecos-neo composable noise system provides event-driven implementation experience. Neither is treated as a compatibility baseline.

PR #545 supplies the device-neutral Selene conformance foundation. A legacy plugin and an event-driven plugin should share immutable parameters and runtime translation, then be compared using:

• independent analytic and qutrit-reference oracles
• multi-seed distributional comparisons
• exact seeded operation/outcome/RNG traces for compatibility mode
• repeated and expanded mutation audits

Review focus

The RFCs end with separate decision checklists. Remaining open questions include:

• leakage scaling and legacy leakage_scale/leak2depolar naming
• channel bulk-method shape, identity-wire scope, and normalization tolerance
• initial exact DEM coverage
• provisional versus first-class Selene trigger transport
• generic trigger metadata serialization
• initial public exposure of batch and circuit selectors

Validation

• uv run --frozen pre-commit run --all-files
• git diff --check
• reviewed against production GeneralNoiseModel, experimental pecos-neo events, and PR feat(selene): add PECOS general noise error-model plugin #545's conformance design