feat: read gate and changelog features from soothfast.toml

[Verdenroz]

What changed

• Added features to the [gate] table of soothfast.toml, applied to gate, measure and spec gen.
• Added a [changelog] table carrying features and packages, applied to report changelog. features falls back to [gate] features when absent.
• Called gate_config::apply from spec gen, which read no repository config at all before.
• Grew changelog_config to return a ChangelogConfig holding the icons alongside the two new keys, instead of Icons alone.
• Extracted the features precedence into resolve_features in report.rs, so the rule is a pure function with its own tests.
• Documented both tables in docs/gating.md and docs/reports.md.

Fixes #193

Why

A bench target that declares required-features, or a crate whose public API is mostly feature-gated, needs --features on every gate, measure, spec gen and report changelog run. The action grew features and changelog-features inputs in v0.3.1, so CI can be told once. A developer running the CLI by hand still had to remember the same flags on every invocation. In the changelog case a wrong set fails quietly: feature-gated public items drop out of the API surface diff and nothing says they did. The file now carries the defaults, and an explicit --features or -p on the command line still wins over it.

How was this tested

• cargo test -p cargo-soothfast: 213 passed, 1 failed. The failure is invoke::tests::a_plain_directory_in_the_repo_is_not_a_worktree_root, which asserts the workspace root is not a git worktree root. This branch does not touch invoke.rs; the test fails because the run happened from inside a linked worktree. Skipping that one test: 213 passed, 0 failed.
• cargo fmt --all -- --check: clean.
• cargo clippy --workspace --all-targets -- -D warnings: clean.
• Seven new unit tests across the two parsers, covering both new keys, their type errors, an unknown key under [changelog], and the [changelog] and [changelog.icons] tables not bleeding into each other.
• Three new unit tests over resolve_features, one per precedence step: an explicit --features beating both tables, [changelog] features beating [gate] features, and [gate] features filling in when neither of the others does.
• Manually, against this repository's own soothfast.toml. With [changelog] packages = ["soothfast-report"] added, cargo soothfast report changelog with no -p reached the baseline step instead of erroring. Without it the report changelog needs -p PKG error is unchanged. An unknown key under [changelog] errors with the file and line.
• make check and make gate BASE=master were not run. No measured function in soothfast-registry, soothfast-measure or soothfast-docs is touched.

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: bench binaries identical (code and data match) — no measurable change possible
gate: gating backend = walltime
gate: build=08f03a noise_floor=0.07% thresholds: instructions +5% ir +5% walltime +10.0% alloc/size +5% polls/wakes +5%
ok    soothfast_sdk::bench_emit_typescript walltime_median_ns 1787959.0 -> 1787959.0 (+0.0%)
ok    soothfast_sdk::bench_emit_typescript allocs 42930 -> 42930 (allowed <= 45076)
ok    soothfast_sdk::bench_emit_typescript alloc_bytes 2938940 -> 2938940 (allowed <= 3085887)
ok    soothfast_sdk::bench_lower walltime_median_ns 1111545.0 -> 1111545.0 (+0.0%)
ok    soothfast_sdk::bench_lower allocs 23907 -> 23907 (allowed <= 25102)
ok    soothfast_sdk::bench_lower alloc_bytes 2064788 -> 2064788 (allowed <= 2168027)
ok    soothfast_sdk::bench_emit_typescript assert complexity: claimed O(n); growth drift x0.98 over sizes [16, 64, 256] (limit x2.5)
ok    soothfast_sdk::bench_lower assert complexity: claimed O(n); growth drift x1.25 over sizes [16, 64, 256] (limit x2.5)
gate: passed (2 item(s))