[Chore] - Publish a mux-runtime version containing BoxedEnum (unblocks dropping the compiler's git patch)

[DerekCorniello]

Summary

main has runtime changes that are not on crates.io, and Cargo.toml still
says version = "0.5.0" (same as the published release). Everything that
landed with the recursive/collection enum work in muxlang/mux-compiler#309 is
currently reachable only via a git dependency:

• managed Value::BoxedEnum { bytes, clone_glue, drop_glue, cmp_glue } with
  value-semantic Clone/Drop (payload-carrying enums stored in
  list/map/set)
• structural comparison entry points used by enum map keys / set members
• the rc-leak-check feature (Add rc-leak-check feature for exact live-block leak detection #23)

Published mux-runtime 0.5.0 has none of these symbols.

Why this matters now

mux-compiler's root Cargo.toml carries a [patch.crates-io] pointing
mux-runtime at this repo's main branch, precisely because the symbols its
codegen emits calls to do not exist in any published version. That patch is a
development stopgap - [patch.crates-io] is ignored for consumers of a
published crate, so it cannot ship. See the companion issue in mux-compiler.

The work

• Bump version in Cargo.toml (0.6.0 - new public FFI surface, no
  removals).
• CHANGELOG entry covering BoxedEnum, the enum comparison glue, and
  rc-leak-check.
• Verify the full feature list still matches what mux-compiler's
  full_runtime_features() parity test expects (that test reads this
  repo's Cargo.toml directly).
• Publish (maintainer-only step per the release process).

Blocks

Publishing this unblocks dropping the git patch in mux-compiler and pinning a
real version there.

---

Companion: muxlang/mux-compiler#316.

[DerekCorniello]

Closed by #28, but not by publishing - so the title is misleading if you find this later.

The publish was needed only to let mux-compiler drop its [patch.crates-io] block. That coupling is gone: the compiler now consumes this repo as a git dependency pinned to a commit by its Cargo.lock, so merging to main is what makes a change available. crates.io is frozen - versions through 0.5.0 stay published and are not yanked, but no new ones go there.

BoxedEnum, the enum comparison glue and rc-leak-check are all reachable from main today with no publish. Rationale: ADR 0004.