Add parsing, validation, and uniqueness rules for the new
`[implements=<interface>]label` extern name form from the component
model `implements` proposal. An implements name labels an instance
import/export that implements a named interface.

Uniqueness: `[implements=<I>]L` conflicts with bare label `L` and with
`[method]L.L` / `[static]L.L` (the existing l.l edge case), but is
strongly unique from interface names, constructors, and normal
method/static names.

See implements feature:
WebAssembly/component-model#613

DO NOT MERGE until wasm-tools release with
bytecodealliance/wasm-tools#2453
Points wasm-tools to PR branch  `wasmparser-implements`

Add support for the component model `[implements=<I>]L`
(spec PR [bytecodealliance#613](WebAssembly/component-model#613)),
which allows components to import/export the same
interface multiple times under different plain names.

A component can import the same interface twice under different labels,
each bound to a distinct host implementation:

```wit
import primary: wasi:keyvalue/store;
import secondary: wasi:keyvalue/store;
```

Guest code sees two separate namespaces with identical shapes:

```rust
let val = primary::get("my-key");       // calls the primary store
let val = secondary::get("my-key");     // calls the secondary store
```

From the host, wit-bindgen generates a separate Host trait per label:

```rust
impl primary::Host for MyState {
    fn get(&mut self, key: String) -> String {
        self.primary_db.get(&key).cloned().unwrap_or_default()
    }
}

impl secondary::Host for MyState {
    fn get(&mut self, key: String) -> String {
        self.secondary_db.get(&key).cloned().unwrap_or_default()
    }
}

primary::add_to_linker(&mut linker, |state| state)?;
secondary::add_to_linker(&mut linker, |state| state)?;
```

The linker also supports registering by plain label without knowing the annotation:

```rust
// Component imports [implements=<wasi:keyvalue/store>]primary
// but the host just registers "primary" — label fallback handles it
linker.root().instance("primary")?.func_wrap("get", /* ... */)?;
```

Users can also register to the linker with the full encoded `implements` name

```rust
let mut linker = Linker::<()>::new(engine);

linker
    .root()
    .instance("[implements=<wasi:keyvalue/store>]primary")?
    .func_wrap("get", |_, (key,): (String,)| Ok((String::new(),)))?;
```

Semver matching works inside the implements annotation, just like regular interface imports:

```rust
// Host provides v1.0.1
linker
    .root()
    .instance("[implements=<wasi:keyvalue/[email protected]>]primary")?
    .func_wrap("get", |_, (key,): (String,)| Ok((String::new(),)))?;

// Component requests v1.0.0, matches via semver
let component = Component::new(&engine, r#"(component
    (type $store (instance
        (export "get" (func (param "key" string) (result string)))
    ))
    (import "[implements=<wasi:keyvalue/[email protected]>]primary" (instance (type $store)))
)"#)?;
linker.instantiate(&mut store, &component)?; // works, 1.0.1 is semver-compatible with 1.0.0
```

## Changes

### Runtime name resolution

- Add three-tier lookup in NameMap::get: exact → semver → label fallback
- Add implements_label_key() helper for extracting plain labels from `[implements=<I>]L`
- Add unit tests for all lookup tiers

### Code generation for multi-import/export

- Track first-seen implements imports/exports per `InterfaceId`
- Duplicate imports: re-export types via `pub use super::{first}::*`,
  generate fresh Host trait + add_to_linker
- Duplicate exports: same pattern with fresh Guest/GuestIndices,
  plus regenerate resource wrapper structs to reference the local Guest type
- Use `name_world_key_with_item` for export instance name lookups
- Guard `populate_world_and_interface_options` with `entry()` to avoid
  overwriting link options for duplicate interfaces

DO NOT MERGE until wasm-tools release with
bytecodealliance/wasm-tools#2453
Points wasm-tools to PR branch  `wasmparser-implements`

Add support for the component model `[implements=<I>]L`
(spec PR [bytecodealliance#613](WebAssembly/component-model#613)),
which allows components to import/export the same
interface multiple times under different plain names.

A component can import the same interface twice under different labels,
each bound to a distinct host implementation:

```wit
import primary: wasi:keyvalue/store;
import secondary: wasi:keyvalue/store;
```

Guest code sees two separate namespaces with identical shapes:

```rust
let val = primary::get("my-key");       // calls the primary store
let val = secondary::get("my-key");     // calls the secondary store
```

Host Import-side codegen: shared trait + label-parameterized add_to_linker

For imports, wit-bindgen generates one Host trait per interface (not per
label). The add_to_linker function takes a name: &str parameter so the
same trait implementation can be registered under different instance labels.
Duplicate implements imports don't generate separate modules — only the
first import produces bindings.

```rust
struct PrimaryBackend;
impl primary::Host for PrimaryBackend {
    fn get(&mut self, key: String) -> String {
        self.primary_db.get(&key).cloned().unwrap_or_default()
    }
}

struct SecondaryBackend;
impl primary::Host for SecondaryBackend {
    fn get(&mut self, key: String) -> String {
        self.secondary_db.get(&key).cloned().unwrap_or_default()
    }
}

// Same add_to_linker, different labels and host_getter closures
primary::add_to_linker(&mut linker, "primary", |s| &mut s.primary)?;
primary::add_to_linker(&mut linker, "secondary", |s| &mut s.secondary)?;
```

Export-side codegen: per-label modules with shared types

For exports, each label gets its own module with fresh Guest/GuestIndices
types but re-exports shared interface types from the first module via
`pub use super::{first}::*`.

Runtime name resolution

The linker supports registering by plain label without knowing the annotation:

```rust
// Component imports [implements=<wasi:keyvalue/store>]primary
// but the host just registers "primary" — label fallback handles it
linker.root().instance("primary")?.func_wrap("get", /* ... */)?;

Users can also register to the linker with the full encoded implements name:

linker
    .root()
    .instance("[implements=<wasi:keyvalue/store>]primary")?
    .func_wrap("get", |_, (key,): (String,)| Ok((String::new(),)))?;
```

Semver matching works inside the implements annotation, just like
regular interface imports:

```rust
// Host provides v1.0.1
linker
    .root()
    .instance("[implements=<wasi:keyvalue/[email protected]>]primary")?
    .func_wrap("get", |_, (key,): (String,)| Ok((String::new(),)))?;

// Component requests v1.0.0, matches via semver
let component = Component::new(&engine, r#"(component
    (type $store (instance
        (export "get" (func (param "key" string) (result string)))
    ))
    (import "[implements=<wasi:keyvalue/[email protected]>]primary" (instance (type $store)))
)"#)?;
linker.instantiate(&mut store, &component)?; // works, 1.0.1 is semver-compatible with 1.0.0
```

- Add three-tier lookup in NameMap::get: exact → semver → label fallback
- Add implements_label_key() helper for extracting plain labels from
  `[implements=<I>]L`
- Add unit tests for all lookup tiers

- Track first-seen implements imports per `InterfaceId`
- One `Host` trait per interface; `generate_add_to_linker` takes
  `named: bool` — when true, emits `name: &str` parameter instead of
  hardcoding the instance name
- Duplicate `implements` imports: just record the label in
  `implements_labels`, no module generation
- `world_add_to_linker`: iterate over `implements_labels` to emit one
  `add_to_linker` call per label, passing label as name argument
- Guard `populate_world_and_interface_options` with `entry()` to avoid
  overwriting link options for duplicate interfaces

- Duplicate exports: re-export types via `pub use super::{first}::*`,
  generate fresh `Guest`/`GuestIndices`, plus regenerate resource wrapper
  structs to reference the local `Guest` type
- Use `name_world_key_with_item` for export instance name lookups