feat(sdk)!: add case_mode: smart, sensitive, insensitive option

[gustav-fff]

Split out from #561 per @dmtrKovalenko request.

What

Adds fff::CaseMode { Smart, Sensitive, Insensitive } and a new optional case_mode: Option<CaseMode> field on GrepSearchOptions. Adds GrepSearchOptions::effective_case_mode() to resolve the effective mode.

Non-breaking

The legacy smart_case: bool field is preserved. When case_mode is None, the existing boolean is used (true => Smart, false => Sensitive) — identical behavior to before. New callers that need Insensitive set case_mode = Some(CaseMode::Insensitive).

No public function signatures changed. Default still emits smart_case: true, case_mode: None. Existing consumers (fff-c, fff-nvim, benches, tests) compile unchanged.

Why this shape

The original PR #561 changed smart_case: bool to case_mode: CaseMode directly, which is a breaking field change for anyone constructing GrepSearchOptions. This PR adds the new API alongside the old one so #561 (and future consumers) can adopt case_mode without a major version bump.

How verified

cargo check -p fff-search passes. Internal call sites in grep.rs route through effective_case_mode() so behavior is preserved when only smart_case is set.

Co-authored-by: Marko Vejnovic contact@markovejnovic.com

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[dmtrKovalenko]

@gustav-fff you need to make sure that it is getting compiled everywhere and mark

[dmtrKovalenko]

@gustav-fff also make it configurable for neovim lua

[gustav-fff]

[triage-bot] DIRECTED: pushed 2ecd02d.

• Added case_mode: None to every GrepSearchOptions construction site across fff-c, fff-mcp, fff-nvim, fff-python, and the integration/fuzz tests so cargo check --workspace --all-targets is clean.
• Exposed case_mode to neovim lua: new optional grep.case_mode = "smart"|"sensitive"|"insensitive" in lua/fff/conf.lua, plumbed through live_grep FFI in crates/fff-nvim/src/lib.rs:435 (parsed into Option<fff::CaseMode>), lua/fff/picker_ui/grep_renderer.lua:34, and M.content_search opts in lua/fff/main.lua:304.
• Legacy smart_case boolean still works; case_mode overrides when set, matching effective_case_mode() semantics.

README update for the new config option not included — flagging for @dmtrKovalenko since per CLAUDE.md new neovim config options should be documented there. Want me to add a section?

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