feat: add JSON streaming encode support

[VimCommando]

Linked Issue

None

Description

Adds an optional json_stream feature to the crate. When enabled you can use serde's streaming deserialization on large JSON objects, concurrently passing chunks of the JSON from the Reader to the Writer. This completely avoids holding the entire input or output in memory and dramatically reduces both memory usage by ~90% and execution time by ~80% (comparison benchmark below).

I've found the TOON format is not only better for LLMs, but is superior to both JSON and YAML for storing and transmitting large data sets. I frequently have 300MB+ JSON files and sometimes just converting to TOON can shave off up to 30% before compression; or it can pretty-print in the same size as a compact single-line JSON.

I have used Codex to assist with this PR and have reviewed the code, following design patterns of my own prior work.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test coverage improvement

Changes Made

• Adds a new crate feature json_stream
• Adds the src/encode/json_stream.rs module
• Adds streaming serialization/writer for the TOON output
• Adds an example to the README.md

SPEC Compliance

• This PR implements/fixes spec compliance
• Spec section(s) affected:
• Spec version: N/A

Testing

• All existing tests pass
• Added new tests for changes
• Tests cover edge cases and spec compliance

Output is byte-for-byte identical to the existing code path.

There is one open cargo clippy warning, but it is unrelated to this PR.

Pre-submission Checklist

• My code follows the project's coding standards
• I have run code formatting/linting tools
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed
• I have reviewed the TOON specification for relevant sections

Breaking Changes

• No breaking changes
• Breaking changes (describe migration path below)

Additional Context

TOON Rust vs TOON Rust Stream

Metric	TOON RS	TOON Stream	Change	% Change
Real time	76.8s	9.03s	-67.8s	-88.2%
User time	74.7s	8.37s	-66.3s	-88.8%
Sys time	1.60s	0.20s	-1.40s	-87.5%
Max RSS	9,922 MB	36.7 MB	-9,885 MB	-99.6%
Peak footprint	7,857 MB	28.8 MB	-7,828 MB	-99.6%

TOON Javascript vs TOON JSON Stream

Metric	TOON JS	TOON Stream	Change	% Change
Real time	264.4s	9.03s	-255.3s	-96.6%
User time	110.9s	8.37s	-102.5s	-92.5%
Sys time	171.6s	0.20s	-171.4s	-99.9%
Max RSS	1,325 MB	36.7 MB	-1,288 MB	-97.2%
Peak footprint	1,353 MB	28.8 MB	-1,324 MB	97.9%

File Summary

File	Size	Bytes	Lines	Characters
segments.json	388M	406,585,264	12,715,306	406,585,264
segments-js.toon	285M	298,411,834	9,682,960	298,411,834
segments-rs.toon	286M	299,718,398	9,682,960	299,718,398
segments-rss.toon	286M	299,718,398	9,682,960	299,718,398

Note
segments-rs.toon and segments-rss.toon are bye-for-byte-identical.

The Javascript output is slightly smaller because it omits quotes on some values that Rust keeps quoted. Example:

<               version: 9.11.1
---
>               version: "9.11.1"