feat(acp): update schema to 1.11.0

[benbrandt]

Summary

• Update the exact agent-client-protocol-schema pin from 1.10.2 to 1.11.0 and refresh only that package's lockfile entry.
• Remove the former unstable notices and compaction feature flags, since both surfaces are now stable, and remove the obsolete compaction-test gate.
• Add compatibility coverage for stable notice routing, strict acted-on session lists returning invalid parameters, null-as-empty list decoding, invalid v2 replay cursors, v2 error/custom stop reasons, and unchanged v1 prompt-error behavior.
• Update the existing Cargo-features and draft-v2 book chapters for the release.

Existing typed routing supports the new schema without additional production routing changes. Removal of the two former unstable feature flags is intentional; callers should remove them from their Cargo feature lists.

Validation

• just test (rerun after removing the former unstable flags): workspace fixture binaries/examples build, unit/integration tests, and doctests
• cargo fmt -- --check
• mdbook build after mdbook-mermaid install .
• git diff --check
• During the upgrade, workspace all-feature Clippy with warnings denied and targeted core no-default-feature/draft-v2 feature checks also passed

The full feature-powerset, MSRV, cross-target, and docs.rs CI matrix was not run. Existing ignored doctests remain ignored; the local book build reports a non-blocking mdbook/Mermaid version warning.