feat: @sse-until predicate evaluator (core slice of #46)

[rwrife]

Part of #46 — next shippable slice on top of the SSE frame parser (#49).

What

Pure, sandboxed predicate evaluator for the # @sse-until <expr> directive at src/core/sse/until.ts. Given a dispatched SseEvent and stream context, evaluates a single-line JS expression and reports whether the stream should stop.

Highlights

• evaluateSseUntil(expr, event, ctx) — one-shot evaluator.
• SseUntilGate — stateful helper that tracks count / index / elapsedMs across events, latches on first truthy result, and captures (but does not act on) predicate errors so a broken expression can't silently truncate a stream.
• Bindings exposed: event (frozen, with type/data/id/retry/json), plus flat aliases data, text, json, type, id, retry, index, elapsedMs, count.
• JSON payload auto-parsed into json when data starts with {/[/" — OpenAI-style json.choices[0].finish_reason === 'stop' and [DONE] sentinels both work.
• Sandbox mirrors src/core/assertions.ts: shadows globalThis, process, require, Function, eval, timers, fetch, etc.; exposes only the standard safe globals (Math, JSON, Date, primitive wrappers, RegExp, parse*).
• No VS Code, no undici, no I/O — fully unit-testable.
• 27 vitest cases covering: JSON parsing (valid / non-JSON / malformed / empty), event binding + aliases, count/index/elapsedMs, id/retry, empty/compile/runtime errors, sandbox shadowing, event view freezing, and SseUntilGate state transitions with an injectable clock.

Not in this PR (still under #46)

• Response viewer streaming mode
• undici wire-up + reconnect / Last-Event-ID header
• .sse.jsonl transcript persistence
• README section
• Directive-level plumbing (parser.ts already collects directives generically; this PR ships the evaluator the transport will call)

Checks

• npm run lint ✅
• npm run typecheck ✅
• npm run test:unit ✅ (458 passed, 27 new)