Fix env_exists for environment values longer than one byte

## Summary

`std::env::env_exists` currently uses a two-byte destination buffer and treats every negative `env_get` result as “not present”. Any environment value longer than one byte therefore produces the buffer-too-small result and is incorrectly reported as absent.

## Relevant code

- `std/env/environ.wave`
- `std/env/parse.wave`

## Scope

Make presence detection independent of the destination capacity needed to copy the value. Preserve the existing public API and integer error conventions unless a small internal helper is needed.

## Acceptance criteria

- A missing key returns `false`.
- An empty value returns `true`.
- One-byte and multi-byte values return `true`.
- Presence detection does not require copying the full value.
- Regression coverage is added to the Wave test corpus.
- Existing standard-library examples and native CI targets continue to pass.

Start by reading std/env/environ.wave and std/env/parse.wave to trace env_exists and the existing env_get error conventions. Run the relevant Wave standard-library tests before changing anything, then add regression coverage to the Wave test corpus. Done means missing, empty, one-byte, and multi-byte values all report the specified presence result without copying the full value, while existing examples and native CI targets pass.

Written by the indexing model from the issue text.