DVM v10 language: structured control flow, subroutines, arrays, mapkeys, Bool, Int, CONST (fixes #125)

[liqdmetal]

Summary

One reviewable PR replacing seven stacked PRs (#101, #103, #107, #109, #111,
#115, #117). The complete DVM v10 language: structured BASIC.

What's in it

Item	Syntax	What it gives
L1	FOR x = a TO b [STEP z] / NEXT, WHILE/WEND, block IF/ELSE/ENDIF	structured loops replace GOTO spaghetti
L2	GOSUB <line> / RETURN	shared helper reuse inside one contract
L3	DIM a(n) AS type, a[i], arrlen(a)	list semantics + loops over arrays
L4	mapkeys()	enumerate SC state keys (batch/paged contracts)
L6	DIM b AS Bool, TRUE/FALSE, &&/`
L7	CONST name = value	immutable named constants
L8	enforced version gate	new syntax hard-requires version("10.0.0")
L9	DIM i AS Int, negative literals, signed arithmetic	signed 64-bit ints (deltas, negatives)

Design notes (why it's safe)

• All gated >=10.0.0 — pre-fork contracts can never accidentally parse new
  syntax (gateControlFlow rejects versions without version()).
• Arrays (*[]Variable), Bool, Int are Locals-only — never serialized to
  RamStore/storage, so the change is consensus-neutral on the state tree.
• Variable.Array is a pointer so Variable stays comparable (it's a map key).
• Block-IF coexists with the existing single-line IF THEN GOTO (untouched).
• RETURN pops the CallStack when non-empty (subroutine return), otherwise
  the existing function-return semantics are unchanged.

Tests

14 files, 8 control-flow test suites (L1–L9) covering: FOR sum/step/nesting,
WHILE countdown, IF/ELSE both branches, nested GOSUB, array fill via loop +
get/set + arrlen, Bool logic + params, CONST immutability, Int negative
arithmetic, and version-gate rejection. Plus a wargame test.

Supersedes

Closes #101, #103, #107, #109, #111, #115, #117 in favour of this single
package.