Add OPEN/CLOSE with both modern (OPEN "f" FOR OUTPUT AS #n) and compact
(OPEN "O",#n,"f") syntaxes. PRINT#, WRITE#, INPUT#, LINE INPUT# for
sequential file access. EOF() now returns real file status with peek-ahead.
LOC/LOF return approximate values.
PRINT USING with numeric (#, ., +, -, $$, **, ^^^^, comma grouping) and
string (!, &, \ \) format specifiers. Shared by PRINT USING and PRINT# USING.
SAVE (ASCII), LOAD (with ,R auto-run), and MERGE for program persistence.
MID$ assignment (MID$(var$, start [,len]) = expr) for in-place string
modification. Works with both scalar variables and array elements.
Graphics stubs for SCREEN, PSET, PRESET, LINE, CIRCLE, DRAW, PAINT,
VIEW, WINDOW, PALETTE - parse and discard arguments so graphics-heavy
programs don't crash.
SYSTEM and NEW/CLEAR now close all open files. Version bumped to 0.3.0.
22 tests pass (16 existing + 6 new).
