U2: Add horror state, action trace, and tests

[breadback00-dev]

Parent: #16

Purpose

Derive horror phase and player implication from game actions in one testable place.

Scope

• Add session-local horror state: action trace, consent pressure, safety/intensity settings, viewed variants, and debrief inputs.
• Add pure helper functions for investigation style, escalation phase, pressure level, and debrief mode.
• Record major player actions: evidence opened, tags applied, verification run, intrusive choice selected, reconstruction submitted, custody chosen.
• Add Node tests for pure helper coverage.

Target files

• Case Box 001 - The Creator Who Left But Kept Posting/src/scripts/horror-engine.js
• Case Box 001 - The Creator Who Left But Kept Posting/src/scripts/case-engine.js
• Case Box 001 - The Creator Who Left But Kept Posting/src/scripts/app.js
• Case Box 001 - The Creator Who Left But Kept Posting/tests/horror-engine.test.js
• Case Box 001 - The Creator Who Left But Kept Posting/package.json

Acceptance checks

• Opening evidence appends a trace entry without duplicating on render.
• Different evidence/tagging patterns produce different investigation-style summaries.
• Verification and invasive choices increase Archive Pressure.
• Reset clears horror state, trace, consent choices, and debrief inputs.
• Helpers return neutral phase when horror content is absent.