Add layered integration test framework with simulated and real-node tiers

[carlaKC]

Adds an integration test framework covering sim-ln's node backends, payment modalities and configuration surface, structured as independent layers:

• Environment layer (integration-tests/src/env/): provisions a network and emits its part of the sim.json config. Two providers: a deterministic in-process simulated graph, and a dockerized heterogeneous regtest network (bitcoind + one node each of LND, CLN, Eclair and ldk-server in a ring of announced channels, via testcontainers). Knows nothing about payments.
• Scenario layer (src/scenario.rs): describes activity (defined/random) and config style (alias vs pubkey references, scalar vs [min, max] values). Knows nothing about provisioning.
• Runner + assertions (src/runner.rs, src/asserts.rs): assembles real config files, drives them through the same public entry points the sim-cli binary uses, and asserts over parsing results, dispatched payments and the results CSV.

Test tiers

Simulated (tests/sim_matrix.rs, runs in the existing test CI job, ~0.2s on virtual time): the full cross-product of node reference styles and value shapes for defined activity, random activity with/without exclusions, seeded determinism, count- and time-bounded runs, untagged connector inference, and negative validation cases asserting specific errors.

Real nodes (tests/real_nodes.rs, #[ignore], new CI job / make integration-real, ~8 min): one shared network, scenarios run sequentially — defined keysends across every directed ring edge (each connector proves it sends and receives cross-implementation, with receipts checked against each destination node's own books), a multi-hop route, alias-referenced activities, and random activity.

Error resistance

Every real-node startup step polls with capped exponential backoff and names the node/step in its failure. Container logs are dumped on startup or scenario failure. Readiness keeps mining while waiting for channels because some implementations (ldk-node) broadcast funding transactions asynchronously after the open call returns.

Notes

• ldk-server has no published image: the harness (and CI, with a docker layer cache) builds one from the upstream repo at the rev the ldk-server-client dependency already pins; SIMLN_LDK_SERVER_IMAGE overrides.
• ACINQ ships no versioned eclair tags past 0.8.0, so eclair is pinned by digest; the image is built from the post-release dev commit and needs -Declair.allow-unsafe-startup=true (a static dev-build guard, nothing dynamic).
• testcontainers is pinned to =0.25.0, the last release whose MSRV fits the repo's Rust 1.85 toolchain; serde_with/time are pinned in Cargo.lock for the same reason.
• Count-bounded assertions tolerate the loss of the final payment record per activity: meeting a payment count triggers shutdown in the same instant as the last dispatch, and the results consumer's select! prefers the shutdown listener over draining pending results. Possibly worth an upstream fix (drain the channel before exiting) — happy to file separately.

Both tiers run green locally on macOS (Apple Silicon, Docker Desktop; eclair runs under Rosetta since its image is amd64-only).

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