fix(cargo-gamma): include Some value mutants by default

[chinayangxiaowei]

Problem

fn_value.some was excluded from @default, so a default run did not generate Some(value) whole-body mutants for functions returning Option<T>. Users had to add --mutators @default,fn_value.some explicitly.

Change

• Include fn_value.some in the default selection.
• Keep @pedantic as a focused alias for the same mutator, preserving scripts that select it.
• Update the engine and CLI design and user documentation, and assert that default selection generates Some(0) and Some(1) for an Option<u8> return.

Validation

• cargo fmt --all -- --check passed.
• cargo test --locked -p cargo-gamma-engine --lib passed: 811 tests.

This PR contains one independent default-selection change. It does not include the macOS, workspace, regex, or reference-return patches in #240–#243.

[Copilot]

🟡 Changes recommended

The sample configuration still contains obsolete guidance to add @pedantic for non-default mutations.

1 open finding

• Update sample config to match @​pedantic mutation guidance · New

What changed in this PR

Enables fn_value.some mutants by default while retaining @pedantic as a focused alias.

Changes:

• Adds fn_value.some to the default mutator catalog.
• Tests default Some(0) and Some(1) generation.
• Updates engine and CLI documentation.

File	Description
crates/​cargo-gamma/​src/​main.rs	Updates CLI documentation.
crates/​cargo-gamma/​README.md	Updates generated user documentation.
crates/​cargo-gamma/​docs/​MUTATORS.md	Documents default and preset membership.
crates/​cargo-gamma/​docs/​DESIGN.md	Records CLI selection behavior.
crates/​cargo-gamma/​docs/​CONFIG.md	Updates configuration guidance.
crates/​cargo-gamma-engine/​src/​ops/​registry/​selection.rs	Tests default and pedantic selection.
crates/​cargo-gamma-engine/​src/​ops/​registry/​catalog.rs	Enables the mutator by default.
crates/​cargo-gamma-engine/​src/​ops/​collect/​tests.rs	Adds default-selection regression coverage.
crates/​cargo-gamma-engine/​docs/​DESIGN.md	Records engine behavior.

🧠 Review effort: Balanced

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[Copilot]

🟢 Approval recommended

The focused registry change is consistently documented and covered by tests for selection and generated candidates.

1 open finding

• Update sample config to match @​pedantic mutation guidance

🧠 Review effort: Balanced

[Copilot]

🟢 Approval recommended

The focused registry change is consistently documented and covered by selection and candidate-generation tests.

0 open findings

1 resolved since last review

• Update sample config to match @​pedantic mutation guidance

🧠 Review effort: Balanced

[Martin Taillefer (geeknoid)]

This was intentional, this mutator is too noisy to be on by default.

[chinayangxiaowei]

Thanks for clarifying the intention. I approached this from the broader mutation coverage I was used to with cargo-mutants, and I understand the concern about noise in the default set.

I'm happy to keep fn_value.some opt-in upstream and enable it explicitly for my own runs with --mutators @default,fn_value.some. Thanks for explaining the tradeoff.