feat(dvm): C6 call_sc + incoming-value forward (fixes #118)

[liqdmetal]

Summary

C6 from the improvement agenda — cross-contract calls: call_sc.
The single biggest DeFi gap in DERO's DVM is closed: a contract can now
call another contract. No more monolithic dApps.

The primitive

LET ok = call_sc(TARGET_SCID, "Deposit", "amount", ITOA(amount), "blind", blind)
IF ok != 0 THEN GOTO fail

• call_sc(scid_hex String, entrypoint String, name1, val1, name2, val2, ...) -> Uint64
• Runs the target's entrypoint in a nested DVM invocation sharing the
  same tx: one gas budget, one commit point, deferred transfers intact
• Returns the callee's RETURN value (0 = success, nonzero = error/rollback)

Execution semantics (the safe minimal version)

Concern	Design
Storage isolation	Each SC lives in its own per-SCID graviton tree; the nested call switches SCIDSELF + Chain_inputs.SCID to the target and resolves/executes against the target's own tree
Atomicity	Nested writes commit to the target's tree directly, safe under the snapshot — a failing top-level tx rolls back the whole chain. Nonzero RETURN rolls back the callee's writes; parent sees it
Reentrancy	Caller's writes are uncommitted until the whole chain succeeds, so a reentrant call sees pre-commit state (checks-effects-interactions, enforced structurally)
Recursion	Monitor_recursion cap of 8 (the codebase's own comment says 64 is already excessive); exceeding it fails the call cleanly, never panics the parent
Auth	Signer propagated unchanged; callee must do its own auth via verify_sig (K0 Fix C) — never trusts "who called me"
Gas	call_sc base cost 10,000 + the callee's execution on the shared budget

Deliberately NOT in v1

• No dynamic dispatch (call-by-SCID only — a registry/interface standard layers on top later)
• No cross-SC value return (funds flow one direction per call)
• No caller-identity to the callee beyond the shared signer

Changes

• dvm/dvm_functions.go — dvm_call_sc handler + func_table entry (>= 10.0.0)
• dvm/dvm.go — Shared_State gains TreeCache + Snapshot (to reach any SC's tree)
• dvm/sc.go — Execute_sc_function carries the tree cache/snapshot through
• dvm/simulator.go, blockchain/transaction_execute.go — wire the tree cache/snapshot

Tests (dvm/call_sc_test.go)

Test	Verifies
TestC6_NestedSuccess	caller → counter's Inc(7) → count persisted = 7
TestC6_FailureRollback	failing callee's write (poison) discarded
TestC6_RecursionCap	self-call chain exceeds depth, fails cleanly

Full dvm suite green; whole tree builds.

Why this matters

Router, escrow, and token can now be separate audited contracts calling
each other instead of one giant contract where a bug in any piece
compromises all of it. Standard interfaces become possible. Upgrade
layering (swap an oracle behind a stable interface) becomes possible.
This is the composability primitive that a settlement rail — atomic swaps,
DEX, confidential AMM, perpDEX — is built on.

Relationship

• Implements research-docs/cross-contract-calls.md (issue Cross-contract calls (CALL_SC) — design proposal #87)
• Stacks on the DVM v9 intrinsics (PR DVM v9 intrinsics: signature verification, Pedersen commitments, asset balance, point addition (fixes #83) #84); sequencing per design: verify_sig first → call_sc → registry/interface standard (next)

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Branch: feature/dvm-c6-callsc in the fork liqdmetal/derohe-improvements-by-liqdmetal (stacked on feature/dvm-v9-intrinsics).

[liqdmetal]

Folded local maturity: call_sc now forwards incoming DERO/asset via reserved args value/assetvalue/asset. Callee sees DEROVALUE()==forwarded amount; insufficient funds returns 2 and rolls back. TestC6_ValueForward. Branch is flat on community-dev (no intrinsic dep).