Phase 3: file I/O, PRINT USING, SAVE/LOAD, MID$ assignment, graphics stubs

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).

tests/programs/write_input.bas

@@ -0,0 +1,11 @@
+10 REM WRITE#/INPUT# and EOF test
+20 OPEN "/tmp/gwbasic_wi_test.txt" FOR OUTPUT AS #1
+30 WRITE #1, "Alice", 25
+40 WRITE #1, "Bob", 30
+50 CLOSE #1
+60 OPEN "/tmp/gwbasic_wi_test.txt" FOR INPUT AS #1
+70 WHILE NOT EOF(1)
+80 INPUT #1, N$, A
+90 PRINT N$; A
+100 WEND
+110 CLOSE #1