feat: add headerOverrides for surgical emulation header replacement

[DepsCian]

Addresses #145 (second part)

Problem

Users who want to replace specific emulation-injected headers (e.g. override Accept while keeping User-Agent, Accept-Language, etc.) currently have no option besides disableDefaultHeaders: true — which strips all emulation headers. This forces manual reconstruction of the entire header set, defeating the purpose of browser fingerprinting.

Solution

Add headerOverrides?: HeadersInit to RequestInit and RequestOptions. Headers listed in headerOverrides replace matching emulation-injected headers by name, while all other emulation headers are preserved.

const response = await fetch("https://example.com", {
  browser: "chrome_142",
  headerOverrides: [
    ["Accept", "text/html"],
    ["Accept-Language", "en-US"],
  ],
});
// User-Agent, Accept-Encoding, Sec-CH-UA, etc. still come from chrome_142
// Only Accept and Accept-Language are replaced

How it works

JS layer — headerOverrides is converted to HeaderTuple[] and threaded through NativeRequestOptions to the Rust native binding.

Rust layer — RequestOptions.header_overrides: Vec<(String, String)> is applied in make_request_inner() after headers, using the same request.header(key, value) call which replaces by name. Both headers and header_overrides share a single orig_headers pass to preserve casing.

Changes

File	Change
rust/src/client.rs	Add header_overrides field to RequestOptions, merge with headers in single orig pass
rust/src/lib.rs	Parse headerOverrides from JS object into RequestOptions
src/types.ts	Add headerOverrides?: HeadersInit to RequestInit and RequestOptions
src/wreq-js.ts	Add to NativeRequestOptions, thread through fetch() and legacy request()
src/test/http/headers.spec.ts	2 new integration tests

Test Results

163/163 pass (full suite with local HTTP server)

New tests:

• headerOverrides replaces specific emulation headers — verifies Accept is overridden while User-Agent is preserved
• headerOverrides with headers coexist — verifies custom headers + overrides + emulation all work together

vs disableDefaultHeaders

	disableDefaultHeaders: true	headerOverrides
Emulation headers	All stripped	Only specified ones replaced
Custom headers	Must reconstruct everything	Add alongside as usual
Fingerprint integrity	Broken (no UA, no encoding)	Preserved (only named overrides)