fix(source-routing): preserve execution-zone read diagnostics

[3esmit]

Summary

Refs #405.

Source-routing adapters previously reduced every execution-zone read error to a category, so Basecamp and standalone could report only a generic health, identity, or head failure. The adapter now retains a bounded causal diagnostic and carries it into unavailable/protocol probe failures.

• Preserve the full error chain up to 512 bytes.
• Keep capability failures on the existing stable unsupported message.
• Keep channel attestation, mismatch rejection, pending-source handling, retries, and endpoint validation unchanged.
• Add regressions for causal propagation, truncation, and capability-message privacy.

Verification

• cargo fmt --all -- --check
• cargo test --locked --lib source_routing::channel_sources::layer --no-fail-fast
• cargo test --locked --lib source_routing::channel_sources::probe --no-fail-fast
• cargo test --locked --lib inspection::l2::source --no-fail-fast
• cargo clippy --locked --lib --all-targets -- -D warnings
• python3 scripts/check-build-pipeline.py ci (passed)
• cargo-semver-checks 0.50.0 against origin/main (223 pass, 31 skip)
• Existing Basecamp and standalone investigate(basecamp): Sequencer identity RPC fails while standalone succeeds #405 acceptance recheck remains green; this change only improves diagnostics on the failure path.

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[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-16T16:21:16.447673Z	97173c3	New commits

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