fix(lambda-rs-args): Optimize parsed argument lookup and positional argument assignment (#189)

## Summary

Speed up `lambda-rs-args` parsing results and improve docs to match
`lambda-rs` rustdoc conventions.

## Related Issues

<!-- Link related issues using "Closes #123" or "Relates to #456" -->

Relates to: (none)

## Changes

<!-- List the key changes made in this PR -->

- Build an index for `ParsedArgs` so `has()` / `get_*()` are O(1) by
name instead of scanning.
- Track a positional cursor so positional assignment after `--` is O(p)
total instead of O(p^2).
- Update rustdoc for the touched APIs/helpers using `# Arguments` / `#
Returns` / `# Errors` / `# Panics` sections.

## Type of Change

<!-- Mark the relevant option(s) with [x] -->

- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] Feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation (updates to docs, specs, tutorials, or comments)
- [x] Refactor (code change that neither fixes a bug nor adds a feature)
- [x] Performance (change that improves performance)
- [x] Test (adding or updating tests)
- [ ] Build/CI (changes to build process or CI configuration)

## Affected Crates

<!-- Mark the crate(s) affected by this change -->

- [ ] `lambda-rs`
- [ ] `lambda-rs-platform`
- [x] `lambda-rs-args`
- [ ] `lambda-rs-logging`
- [ ] Other:

## Checklist

<!-- Ensure the following before requesting review -->

- [ ] Code follows the repository style guidelines (`cargo +nightly fmt
--all`)
- [ ] Code passes clippy (`cargo clippy --workspace --all-targets -- -D
warnings`)
- [ ] Tests pass (`cargo test --workspace`)
- [x] New code includes appropriate documentation
- [x] Public API changes are documented
- [ ] Breaking changes are noted in this PR description

## Testing

<!-- Describe how the changes were tested -->

**Commands run:**

```bash
cargo test -p lambda-rs-args
```

**Manual verification steps (if applicable):**

1. N/A

## Screenshots/Recordings

<!-- For visual changes, include before/after screenshots or recordings
-->

N/A

## Platform Testing

<!-- Mark platforms where changes were verified -->

- [x] macOS
- [ ] Windows
- [ ] Linux

## Additional Notes

Author: vmarcella