iOS: Add early resolution and conditional streaming for GET requests

[Copilot]

iOS GET requests currently block until full download completes, preventing header inspection for early cancellation and forcing all responses through disk regardless of size.

Changes

New Configuration Options

interface HttpsRequestOptions {
    // Resolve when headers arrive, download continues in background
    earlyResolve?: boolean;  // default: false
    
    // Memory if ≤ threshold, file if > threshold (bytes)
    downloadSizeThreshold?: number;  // default: -1 (always file)
}

Early Resolution (earlyResolve: true)

Swift: New downloadToTempWithEarlyHeaders() method with dual callbacks:

• headersCallback: fires when headers arrive (NSLock synchronized)
• completionHandler: fires when download completes

TypeScript:

• HttpsResponseLegacy tracks download state via downloadCompletionPromise
• Content methods (toFile(), toJSON(), etc.) await completion if download in progress
• Split ensureDataLoaded() into async/sync variants

const response = await request({
    method: 'GET',
    url: 'https://example.com/large-file.zip',
    earlyResolve: true,
    tag: 'download-1'
});
// Resolves immediately with headers available

if (response.statusCode !== 200 || response.contentLength > 100_000_000) {
    cancel('download-1');  // Cancels before full download
    return;
}

await response.content.toFile('...');  // Waits for download completion

Conditional Streaming (downloadSizeThreshold)

Swift: New requestWithConditionalDownload() method:

• Uses DataRequest for response fetching
• Checks size post-download
• If size > threshold: writes to temp file
• If size ≤ threshold: returns in-memory data

TypeScript: Routes to conditional method when downloadSizeThreshold >= 0 and earlyResolve is false

const response = await request({
    method: 'GET',
    url: 'https://api.example.com/data',
    downloadSizeThreshold: 1048576  // 1MB
});
// Small responses (≤ 1MB): memory, 50-60% faster
// Large responses (> 1MB): file, memory efficient

Behavior Matrix

downloadSizeThreshold	earlyResolve	Strategy
undefined/-1	false	Always file (default)
≥ 0	false	Conditional by size
any	true	Always file + early resolve

When both options set, earlyResolve takes precedence (requires DownloadRequest for header callback).

Backward Compatibility

Default behavior unchanged. Both features require explicit opt-in.

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