Bun parity: finish Headers inspection and normalization matrix

Tracks the next focused checkpoint under #66, following #372.

Upstream corpus frontier: js/web/fetch/headers.test.ts (position 4158).

Scope:

• preserve the upstream test unchanged
• make Bun.inspect(Headers) retain the Headers brand for empty and populated collections
• keep Bun-compatible normalized ordering and trailing-comma formatting
• keep toJSON(), count, iteration, and SIMD header-name lowercasing behavior logically aligned
• retain actionable, composed error stacks for invalid header operations
• land a permanent regression and validate the contiguous corpus tail plus the full build

Parent: #66 Predecessor: #372

Completed in 5f99cc5 (feat(fetch): preserve Headers inspection branding) and pushed to main.

Validation:

• upstream js/web/fetch/headers.test.ts remains byte-for-byte unchanged
• exact position 4158: 94 passed / 0 failed / 0 todo / 0 unsupported / 0 allowed-empty
• consolidated Bun.inspect(Headers) and the Node custom-inspection symbol behind one branded formatter
• preserved Bun normalization order and multiline trailing-comma output while keeping sorted toJSON(), count, iteration, and SIMD lowercasing semantics separate
• contiguous positions 4159–4163 also pass: 70 passed / 0 failed / 0 todo / 0 unsupported
• zig build --cache-dir /private/tmp/home-zig-abi-cache passes

The contiguous frontier is now position 4164, js/web/fetch/stream-fast-path.test.ts. Parent: #66. Predecessor: #372.