feat: pure .ws parser with zod validation (#40)

[rwrife]

Closes #40 (partial — first slice)

What changed

• New src/core/ws.ts — pure .ws file parser with zero VS Code / undici dependencies. Returns a zod-validated ParsedWebSocketRequest (url, scheme, headers, directives, send/recv frames).
• 13 new vitest cases in test/ws.test.ts covering target parsing, ws/wss schemes, directive + header preamble, interleaved --- send/--- recv frames, multi-line frame bodies, case-insensitive markers, and every error path.

Format shipped

# @name subscribeToTicker
# @auth my-bearer
wss://stream.example.com/v1/ticker?symbol=BTC
Authorization: Bearer {{token}}
Sec-WebSocket-Protocol: json

--- send
{"op":"subscribe","channel":"trades"}
--- recv
{"op":"subscribed"}

Mirrors the .grpc precedent: dedicated file kind, pure parser first, thin extension layer on top later.

What's left for #40 (follow-up PRs)

• src/core/wsClient.ts — undici-backed WS wrapper (thin, no VS Code)
• src/extension/wsView.ts — Preact webview with frame log + send box
• .ws file association + syntax highlighting (reuse header grammar)
• Tree view integration + ▶ Open WebSocket codelens
• Auth reuse (bearer/basic/apiKey/mTLS) on the handshake
• "Save transcript" to .requests/.history/
• README example

Keeping this PR small and reviewable per PLAN.md philosophy.

Verification

• npm run lint ✅
• npm run typecheck ✅
• npm run test:unit ✅ (366 tests, +13)