release: bump workspace to 0.6.0, finalize changelog and docs

[iainmcgin]

Bumps the workspace to 0.6.0, finalizes the CHANGELOG, and corrects documentation that drifted from the merged feature set.

Verified merged PR list since v0.5.2

Everything from the 0.6.0 target is on main:

• feat(buffa-codegen): gate generated json/views/text impls on crate features #117 / feat(buffa-descriptor): regen with views/json/text/arbitrary behind crate features #118 / build: expose gate_impls_on_crate_features in buffa-build and protoc-gen-buffa #119 — feature-gated codegen + buffa-descriptor regen + buffa-build/plugin plumbing
• feat(buffa-codegen): file-level extern path routing for descriptor.proto #114 — file-level extern path routing for descriptor.proto
• fix(buffa): closed-enum JSON helpers decode via Enumeration, not Deserialize #115 — closed-enum bound relaxation
• fix(buffa): re-export serde_json so extension JSON deserialize compiles without a direct consumer dep #112 — serde_json re-export
• view: impl Serialize for generated view types (#83) #106 — view Serialize impls
• fix(buffa-codegen): omit empty ancillary content files #107 — empty ancillary file omission
• codegen: add with_* builder-style setter methods for explicit-presence fields (#30) #93 — with_* builder-style setters (closes Construction ergonomics for explicit-presence fields (edition 2023+) #30)
• feat(buffa): add MessageFullName trait #108 — MessageName trait
• docs: document published BSR plugin and add bsr-quickstart example #116 — BSR plugin docs + examples/bsr-quickstart/
• build: enable lto + codegen-units=1 for release profile #85 — LTO release profile

Changes

Version — workspace 0.5.2 → 0.6.0, propagated to [workspace.dependencies] and the four per-crate Cargo.tomls that hardcode version = "..." on buffa (buffa-build, buffa-codegen, protoc-gen-buffa, protoc-gen-buffa-packaging). Cargo.lock updated.

CHANGELOG — [Unreleased] → [0.6.0] - 2026-05-15, new empty [Unreleased] section, comparison links fixed (added [0.6.0], [0.5.2], [0.5.1] which were missing; rebased [Unreleased] from v0.5.0...HEAD to v0.6.0...HEAD).

Docs accuracy — the highest-priority finding from a docs audit against the v0.6.0 feature set:

• docs/guide.md § Descriptor types said descriptor message types referenced as fields with views/json/text "will not currently compile; please file an issue" — directly contradicting feat(buffa-descriptor): regen with views/json/text/arbitrary behind crate features #118, which made that work. Rewritten to document the actual requirement (enable matching buffa-descriptor features) with a Cargo.toml example and the failure mode you'd see otherwise.
• 11 inline version = "0.5" examples bumped to "0.6" in guide.md; same for the migration guides, README, and examples/bsr-quickstart/Cargo.toml.
• Release-binary download instructions and BSR plugin pin guidance bumped from v0.5.2/v0.5.3 to v0.6.0.
• New paragraph in the JSON section noting view types implement serde::Serialize directly when generate_json is enabled, with the two known limitations from view: impl Serialize for generated view types (#83) #106.

Deliberately not changed

• examples/bsr-quickstart/buf.gen.yaml stays pinned to v0.5.2 — the checked-in src/gen/ was generated by that BSR plugin version, and the v0.6.0 BSR plugin doesn't exist until Buf publishes the bump in bufbuild/plugins (post-release follow-up, see bufbuild-plugins-prs). Update the pin and regenerate together.
• README.md benchmark caption stays "measured at v0.5.0" — benchmarks have not been re-run for 0.6.0.

Suggested follow-ups (not in this PR)

The docs audit also flagged two completeness gaps that aren't inaccuracies but might be worth prose:

1. with_* setters (codegen: add with_* builder-style setter methods for explicit-presence fields (#30) #93) and MessageName (feat(buffa): add MessageFullName trait #108) are documented in the option tables and the custom-type-impl section respectively, but neither appears in "Generated code shape" or the README features list.
2. buffa-build/Cargo.toml, buffa-codegen/Cargo.toml, protoc-gen-buffa/Cargo.toml, and protoc-gen-buffa-packaging/Cargo.toml carry hardcoded version = "0.6.0" constraints on buffa rather than { workspace = true }. Each release bump touches five files instead of one.

Verification

cargo +1.95 fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace, RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps, and task lint-md all pass locally.

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