Release: weavegraph v0.4.0 — Final API Freeze

[Idleness76]

Summary

v0.4.0 is the final API freeze release. No new features — only removals, renames, and documentation finalization. The public API surface is now locked for v1.0.

Previously merged to staging via PR #188 and validated there. This PR promotes staging → main.

Milestones

• 0.4.1 — Removed all deprecated items from 0.2.0/0.3.0: Message::new(), RuntimeConfig.checkpointer field + with_checkpointer() + checkpointer_type(), six legacy AppRunner constructors, LadderError type alias
• 0.4.2 — Removed llm = ["rig"] feature alias; rig is now the sole feature name
• 0.4.3 — Added DIAGNOSTIC_SCOPE const alongside STREAM_END_SCOPE
• 0.4.4 — Added examples/production_streaming.rs: full Axum + SSE + Postgres checkpointing reference, end-to-end tested against live Postgres

Additional

• cargo deny check clean (aws-lc-sys updated 0.38→0.40 to clear 6 security advisories)
• cargo semver-checks clean against 0.3.0 baseline (now running on stable — cargo-semver-checks requires rustc >=1.91.0)
• docs/MIGRATION.md v0.4.0 section added (6 breaking changes with before/after examples)
• CHANGELOG.md [0.4.0] entry added
• Nightly doc lint fixes: 6 rustdoc::redundant-explicit-links errors resolved

Pre-PR Checks (on staging)

• make ci-quick clean (fmt, clippy, lib tests, nightly doc)
• Staging CI: 8 passed, 1 skipped (benchmarks — expected, only runs on main pushes)