feat: add a safe wrapper for shared memory zones

[u5surf]

Proposed changes

Declaring a shared memory zone currently means calling ngx_shared_memory_add by hand, installing an unsafe extern "C" initialization callback, and casting the untyped ngx_slab_pool_t::data pointer back to the payload type at every access. Nothing ties those casts together, so a module using more than one zone can read a zone as the wrong type with no diagnostic.

This adds ngx::core::SharedZone<T>, which keeps the payload type in the signature:

struct SharedDict(RwLock<RbTreeMap<NgxString<SlabPool>, NgxString<SlabPool>, SlabPool>>);

impl SharedZoneData for SharedDict {
    fn new_in(alloc: SlabPool) -> Result<Self, AllocError> {
        Ok(Self(RwLock::new(RbTreeMap::try_new_in(alloc)?)))
    }
}

// directive handler
smcf.shm_zone = Some(SharedZone::add(cf, name, size, MyModule::module())?);

// request handling
let shared = smcf.shm_zone.as_ref().and_then(SharedZone::get)?;

SharedZone::add declares or joins a zone and installs a generic initialization callback; SharedZone::get hands back a &T without a cast. SharedZoneData::reuse is an optional hook for validating a value left behind by a previous configuration, which the raw interface offers no obvious place for.

Two design notes worth reviewing:

• Zone contents are recovered from the slab pool rather than from ngx_shm_zone_t::data, so a zone reused across a configuration reload and a zone inherited from the master process on binary upgrade take the same path.
• Zones declared noreuse are not covered: their mapping is replaced, and carrying data across that boundary needs the data argument this wrapper deliberately leaves alone. Happy to extend this if you would rather it be handled up front.

examples/shared_dict is ported over as the first user, which drops the ten unsafe blocks it needed for zone handling (57 → 48 unsafe in the file overall).

Marked as a draft: the API shape is the part I would like feedback on before polishing.

Testing

• cargo test --lib, cargo clippy --all-targets, cargo fmt --check clean, with and without --features vendored.
• examples/t/shared_dict.t against nginx 1.30.4 built from source: identical results before and after this change (3 pre-existing failures on macOS, unrelated to this PR — they concern updating an existing key in RbTreeMap, and reproduce on main).

Checklist

• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• I have added tests (when possible) that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork