L4: mapkeys — SC state key enumeration for DVM-BASIC

[liqdmetal]

Summary

L4 from the DVM-BASIC language agenda (P1): mapkeys() -> String — map enumeration for SC state. Batch/paged contracts can iterate their stored keys instead of hand-rolling key sets.

The intrinsic

LET ks = mapkeys()     ' "alpha,beta,gamma" — sorted, comma-separated

The effective key set is the union of:

• RamStore — keys loaded from disk (via DiskLoader) during this call
• RawKeys — keys written during this call (TX_Storage.RawKeys)

Keys are the String/Uint64 values of the stored Variables, unmarshaled, deduped, sorted, and joined. The comma-separated String is the DVM-friendly return (intrinsics return String/Uint64); contracts split it for iteration, and L3 arrays give the next natural return type.

Why it matters

Today a contract that batches/pages state must hand-roll its key set (store a counter, maintain a registry). With mapkeys, the keys themselves are enumerable — combined with L1's FOR and L3's arrays, a contract can:

FOR i = 0 TO <n>
    LET k = <split(ks, i)>
    LET v = LOAD(k)
    ... process ...
NEXT i

Consensus safety

• Gated >= 10.0.0 like L1/L2/L3 — pre-fork contracts can't use it
• Read-only: enumerates keys, changes no state

Tests (dvm/control_flow_l4_test.go, 3 cases)

Test	Verifies
TestL4_MapkeysBasic	STORE alpha/beta/gamma → "alpha,beta,gamma" (sorted)
TestL4_MapkeysIterate	batch iteration pattern
TestL4_MapkeysVersionGate	rejected at 1.2.3

Full dvm suite green.

Relationship

• Stacks on L3 (PR L3: RAM arrays for DVM-BASIC (fixes #106, stacks on #103) #107) — same >= 10.0.0 gate; L3 arrays give the next natural return type
• Completes the L-series P1 items (L3 arrays + L4 enumeration) on top of the P0 items (L1 control flow + L2 subroutines)

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Branch: feature/dvm-l4-mapkeys in the fork liqdmetal/derohe-improvements-by-liqdmetal (stacked on feature/dvm-l3-arrays).