feat(worker): add Flagship binding

[connyay]

Adds first-class support for the Flagship feature-flag binding announced on 2026-04-17. Users can now evaluate flags via env.flagship("FLAGS") with a fluent EvaluationContext builder and typed EvaluationDetails<T> responses.

• worker-sys: raw wasm_bindgen externs for all 9 Flagship JS methods
• worker: Flagship wrapper, EvaluationContext builder, EvaluationDetails
• test: mini-flagship miniflare mock + 10 handlers + 16 vitest specs
• examples/flagship: runnable worker demoing value/details/context usage

[connyay]

@guybedford I took an initial swing at ts-gen for flagship.

Mostly worked, but a few maybe controversial things:

(resolved) - promise values that returned primitives had pretty bad ergonomics:

1. promise values that returned primitives had pretty bad ergonomics: ```rust let value = env .flagship(BINDING)? .get_string_value(&flag, "fallback") .await? .as_string() .unwrap_or("fallback") ``` ~Tried this https://github.com/wasm-bindgen/ts-gen/pull/11 which improved consumer ergonomics at the cost of generation complexity and ugliness. let me know what you think.~

EDIT: less bad version wasm-bindgen/ts-gen#14

not too bad:

let value: bool = env
        .flagship(BINDING)?
        .get_boolean_value(&flag, false)
        .await?
        .value_of();

let value = String::from(
        env.flagship(BINDING)?
            .get_string_value(&flag, "fallback")
            .await?,
);

2. I continue to fight with object generic / jsgeneric ergonomics in wasm
   with ootb ts-gen the .get_object_value call looked like:

pub async fn handle_object(req: Request, env: Env, _data: SomeSharedData) -> Result<Response> {
    let flag = last_segment(&req)?;
    let default = serde_wasm_bindgen::to_value(&default_theme())?;
    let raw = env
        .flagship(BINDING)?
        .get_object_value(&flag, &default)
        .await?;
    let value: Theme = serde_wasm_bindgen::from_value(raw)?;
    Response::from_json(&serde_json::json!({ "flag": flag, "value": value }))
}

yuck. fix for this was to comment out the type for generation and handroll. open to ideas to improve this. hit similar stuff in workflows as we've discussed before #918 (comment)

3. the ts-gen _with_record_N generation is pretty nasty.
   what do you think about Collapse Record<K, Union> per-arm explosion to a single binding wasm-bindgen/ts-gen#13

[guybedford]

Thanks for working through the concerns here! This gives a lot of interesting feedback to think about - I will look into this next week and see if I can come up with some further suggestions as well.