Document Zork 1 milestone in if-journey

The hybrid interpreter can now run Zork 1, marking the first working
implementation of the embedded interpreter path. This enables levels 2-5
(inspectable/moldable/shared worlds) rather than just the opaque terminal
approach of level 1.

docs/how/if-journey.rst

@@ -248,6 +248,28 @@ Concrete next steps, roughly ordered. Update as items get done.
 
 - [x] find the game files: locate freely distributable z-machine story files for the games we care about. Wizard Sniffer, Lost Pig, Zork (if legally available). (zork1.z3 bundled in content/stories/)
 
+milestone — Zork 1 playable in hybrid interpreter
+--------------------------------------------------
+
+The hybrid interpreter (zvm architecture + ported viola opcodes) can now run Zork 1. This is the first working implementation targeting levels 2-5 — inspectable, moldable, and shared worlds. Level 1 (terminal mode) uses subprocess dfrotz; this is the embedded path.
+
+What works:
+
+- 69 V3 opcodes ported, all Zork 1-required opcodes implemented
+- key implementations: ``op_test`` (conditional logic), ``op_verify`` (story file checksums), ``sread`` with ZLexer tokenization (parsing player input)
+- ``step()`` method for async MUD integration — single instruction at a time, no blocking loop
+- instruction trace deque (last 20 instructions) for debugging state errors
+- smoke test: ``scripts/run_zork1.py`` runs the game headless, exercises core opcode paths
+
+What this enables:
+
+- Read z-machine state (object tree, variables) from MUD code (level 2)
+- Write z-machine state, inject items, modify world (level 3)
+- Multiplayer instances (level 4, following MojoZork patterns)
+- Entity bridge (level 5, further out)
+
+The step-based execution model means IF sessions can run in the async MUD game loop without blocking. Each player command advances their z-machine instance by N instructions (until output or a stopping condition). The trace deque captures the last 20 instructions for debugging unexpected state.
+
 related documents
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