chore: release v0.3.1

[Verdenroz]

What changed

Cuts v0.3.1. The Unreleased heading becomes ## 0.3.1 - 2026-09-06 with Overview and Upgrade notes written, the workspace version and the eight intra-workspace requirements move from 0.3.0 to 0.3.1, the lockfile follows, and the README dependency snippet and soothfast.toml site version match. Two wording fixes ride along: the 0.3.0 Overview no longer compares soothfast to another product, and a docs/spec.md example uses a placeholder crate name instead of a real project's.

Why

The one-step action in 0.3.0 could not run a bench target that declares required-features, could not diff the changelog over a crate without a bench, and had no way to land derived files outside its built-in regeneration. #187 adds inputs for each and #188 hardens the broker's tag rule against annotated tag objects. Both are action and broker changes, but the release workflow requires the tag to match the workspace version, so they ship as a patch release of the crates too. The frozen section was read against master d5d84f4 after the last changelog regeneration and lists exactly the two merges since v0.3.0.

How was this tested

• make check on this tree (fmt, clippy -D warnings, cargo test --workspace --all-features): exit 0, 25 test binaries green.
• cargo metadata --offline refreshed Cargo.lock; the diff is ten version lines and nothing else.
• The tag push runs release.yml end to end, including the tag-object peel now live in the broker.

Checklist

• make check passes (fmt, clippy -D warnings, cargo test --workspace)
• make gate BASE=master passes, or any intentional cost change is explained above
• Tests added/updated for new behavior (not required for docs/CI-only changes)
• Public API items have /// doc comments
• No new dependency, or its justification is included above (see Dependency Policy)
• Docs (README.md, docs/, soothfast:bind/soothfast:claim markers) updated if behavior changed

[soothfast-bot]

soothfast gate

gate: measuring merge-base of origin/master in worktree (interleaved rounds)
gate: gating backend = callgrind
gate: build=08f03a noise_floor=4.78% thresholds: instructions +5% ir +5% walltime +14.3% alloc/size +5% polls/wakes +5%
NOTE: noise floor 4.78% raised the walltime threshold to +14.3% (deterministic metrics unaffected)
ok    soothfast_spec::bench_openapi_diff instructions 41528349.0 -> 41193511.0 (-0.8%)
ok    soothfast_spec::bench_openapi_diff callgrind_ir 41528349.0 -> 41193511.0 (-0.8%)
ok    soothfast_spec::bench_openapi_diff walltime_median_ns 3133412.0 -> 3080333.0 (-1.7%)
ok    soothfast_spec::bench_openapi_diff allocs 77722 -> 77722 (allowed <= 81608)
ok    soothfast_spec::bench_openapi_diff alloc_bytes 10815487 -> 10815487 (allowed <= 11356261)
ok    soothfast_spec::bench_openapi_document instructions 16803544.0 -> 16801872.0 (-0.0%)
ok    soothfast_spec::bench_openapi_document callgrind_ir 16803544.0 -> 16801872.0 (-0.0%)
ok    soothfast_spec::bench_openapi_document walltime_median_ns 1159899.0 -> 1177496.0 (+1.5%)
ok    soothfast_spec::bench_openapi_document allocs 30152 -> 30152 (allowed <= 31659)
ok    soothfast_spec::bench_openapi_document alloc_bytes 4715065 -> 4715065 (allowed <= 4950818)
ok    soothfast_spec::bench_serialize_yaml instructions 61226343.0 -> 61625690.0 (+0.7%)
ok    soothfast_spec::bench_serialize_yaml callgrind_ir 61226343.0 -> 61625690.0 (+0.7%)
ok    soothfast_spec::bench_serialize_yaml walltime_median_ns 3623112.0 -> 3793967.0 (+4.7%)
ok    soothfast_spec::bench_serialize_yaml allocs 74277 -> 74277 (allowed <= 77990)
ok    soothfast_spec::bench_serialize_yaml alloc_bytes 8533615 -> 8533615 (allowed <= 8960295)
ok    soothfast_spec::bench_openapi_diff assert complexity: claimed O(n); growth drift x1.29 over sizes [16, 64, 256] (limit x2.5)
ok    soothfast_spec::bench_openapi_document assert complexity: claimed O(n); growth drift x1.32 over sizes [16, 64, 256] (limit x2.5)
ok    soothfast_spec::bench_serialize_yaml assert complexity: claimed O(n); growth drift x1.14 over sizes [16, 64, 256] (limit x2.5)
gate: passed (3 item(s))