L3: RAM arrays for DVM-BASIC — DIM a(n), a[i] get/set, arrlen

[liqdmetal]

Summary

L3 from the DVM-BASIC language agenda (P1): first-class RAM arrays — DIM a(n) AS type, a[i] get/set, arrlen. Gives contracts list semantics instead of hand-rolled scalar keys, and composes with L1's FOR loops for iteration.

The syntax

DIM a(10) AS Uint64      ' zero-filled array, indices 0..10 (cap 1024)
LET a[0] = 10
LET a[1] = 20
LET a[2] = a[0] + a[1]   ' expression index works too (loop vars)
RETURN arrlen("a")       ' 11

With L1's FOR:

FOR i = 0 TO 10
    LET a[i] = i * i
NEXT i

Implementation

• Variable gains Array *[]Variable — a pointer so Variable stays comparable (it's used as a map key in RamStore/SC state; a raw slice would break compilation).
• DIM a(n) AS Uint64|String — detects the ( n ) form in interpret_DIM, allocates n+1 zero-filled elements, cap 1024.
• a[i] read — new *ast.IndexExpr case in the evaluator (Go's parser already tokenizes a [ i ]).
• LET a[i] = expr — new branch in interpret_LET; the index is evaluated (literal or expression like a loop variable), with bounds checking.
• arrlen("a") — intrinsic returning the length, gated >= 10.0.0.

Consensus safety

• Arrays are Locals-only: contracts cannot STORE an array (STORE takes scalars), so arrays never touch the serialized SC state — consensus-neutral (no state-format change).
• Gated >= 10.0.0 like L1/L2 — pre-fork contracts can't use it.

Tests (dvm/control_flow_l3_test.go, 4 cases)

Test	Verifies
TestL3_ArrayBasics	set/read/compute + arrlen
TestL3_ArrayForLoop	L1+L3 compose: sum of squares 0..10 = 385
TestL3_StringArray	string arrays
TestL3_VersionGate	DIM a(n) at 1.2.3 rejected

Full dvm suite green.

Relationship

• Stacks on L2 (PR L2: subroutines for DVM-BASIC — GOSUB/RETURN (fixes #102, stacks on #101) #103) — same >= 10.0.0 gate and interpreter machinery
• Enables the next agenda item L4 (map enumeration) — arrays give mapkeys a natural return type

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Branch: feature/dvm-l3-arrays in the fork liqdmetal/derohe-improvements-by-liqdmetal (stacked on feature/dvm-l2-subroutines).