Published crates declare no rust-version but require post-1.90 APIs (floor_char_boundary) — consumers get E0658 instead of a clean MSRV error

[ohdearquant]

Bug

The 0.3.0 crates on crates.io declare no rust-version, but at least khive-pack-brain requires a compiler newer than 1.90: src/persist.rs:388 calls str::floor_char_boundary, which is unstable on 1.90 (round_char_boundary feature).

Consumers building with an older toolchain therefore fail deep inside the dependency compile with a raw feature-gate error:

error[E0658]: use of unstable library feature 'round_char_boundary'
  --> khive-pack-brain-0.3.0/src/persist.rs:388

instead of cargo's clean up-front refusal ("package requires rustc X.Y or newer").

Hit in practice by a downstream Docker build whose builder image pinned rust:1.90-slim-bookworm: every local gate passed (host toolchain 1.94.1), then the container build failed at the dependency.

Suggested fix

Declare rust-version in [workspace.package] in crates/Cargo.toml and inherit it (rust-version.workspace = true) in every published crate. Set it to the true MSRV (the floor_char_boundary/ceil_char_boundary stabilization release, post-1.90; verify against the oldest toolchain that compiles the workspace). A CI job pinned to the declared MSRV would keep the declaration honest as dependencies move.