forth: File Access word set (in-memory backing, Hayes unchanged)

plans/forth-on-sx.md

@@ -93,7 +93,7 @@ Representation:
 - [x] Parsing/dictionary: `WORD`, `FIND`, `EXECUTE`, `'`, `[']`, `LITERAL`, `POSTPONE`, `>BODY` (DOES> deferred — needs runtime-rebind of last CREATE)
 - [x] Source/state: `EVALUATE`, `STATE`, `[`, `]` (`SOURCE`/`>IN` stubbed; tokenized input means the exact byte/offset semantics aren't useful here)
 - [x] Misc Core: `WITHIN`, `MAX`/`MIN` (already), `ABORT`, `ABORT"`, `EXIT`, `UNLOOP`
-- [ ] File Access word set (via SX IO)
+- [x] File Access word set (in-memory — `read-file` is not reachable from the epoch eval env)
 - [ ] String word set (`SLITERAL`, `COMPARE`, `SEARCH`)
 - [ ] Target: 100% Hayes Core
 
@@ -106,6 +106,20 @@ Representation:
 
 _Newest first._
 
+- **Phase 5 — File Access word set (in-memory backing; +4).**
+  `OPEN-FILE`/`CREATE-FILE`/`CLOSE-FILE`/`READ-FILE`/`WRITE-FILE`/
+  `FILE-POSITION`/`FILE-SIZE`/`REPOSITION-FILE`/`DELETE-FILE` plus
+  the mode constants `R/O`/`R/W`/`W/O`/`BIN`. File handles live on
+  `state.files` (fileid → {content, pos, path}) with a
+  `state.by-path` index so `CREATE-FILE`'d files can be
+  `OPEN-FILE`'d later in the same session. Attempting to
+  `OPEN-FILE` an unknown path returns `ior != 0`; disk-backed
+  open/read is not wired because `read-file` isn't in the sx_server
+  epoch eval environment (it's bound only in the HTTP helpers).
+  Also removed the stray base-2 `BIN` primitive from Phase 4 —
+  ANS `BIN` is the file-mode modifier. Hayes Core unchanged at
+  486/638 since core.fr doesn't exercise file words.
+
 - **Phase 5 — `WITHIN`/`ABORT`/`ABORT"`/`EXIT`/`UNLOOP` (+7;
   Hayes 477→486, 76%).** `WITHIN` uses the ANS two's-complement
   trick: `(n1-n2) U< (n3-n2)`. `ABORT` wipes the data/return/control