feat: implement ReadableStream and URL/HTTP input support

[bytebrujo]

Summary

• Implements ReadableStream input: consumes stream chunks, concatenates, and feeds to Zig parser via buffer
• Implements URL input: detects http:///https:// URLs, uses fetch() to download, parses response as buffer
• Adds download: boolean config flag for explicit URL mode
• Adds load() async method for explicit initialization of URL/stream sources
• Async iterator (for await...of) auto-calls load() — no explicit call needed
• Sync iterator throws a helpful error if load() wasn't called for async sources
• load() is idempotent — safe to call multiple times

Test plan

• Parses from single-chunk ReadableStream
• Parses from multi-chunk ReadableStream (data split across chunks)
• Async iterator auto-loads from ReadableStream
• Sync iterator throws helpful error without load() for stream
• Fetches and parses from HTTP URL
• Async iterator auto-loads from URL
• download: true flag treats string as URL
• Throws on HTTP error (404)
• load() is idempotent
• All 37 existing + new tests pass

Closes #10

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