refactor(runtime): use VMPILOT_TRY / TRY_ASSIGN on vm_engine hot path

Six identity-propagation sites in the dispatch loop that used to
read:

    auto x_or = f(...);
    if (!x_or) return tl::make_unexpected(x_or.error());
    auto x = *x_or;

collapse to VMPILOT_TRY or VMPILOT_TRY_ASSIGN. Covers BlobView::create,
pipeline::fetch_decrypt_decode, pipeline::verify_bb_mac,
pipeline::enter_basic_block, dispatch_unit, and VmEngine::create.

Most of the 120 call sites the retrospective estimated are already
gone — schema_parse subsumed the per-field require_* chains. What
remains falls into two groups: pure identity propagation (migrated
here and in the companion commits) and error-code substitution
(e.g.  for a generic BadCbor), which
stays imperative because the macro forces the original error.

Author: scc-tw

runtime/src/vm_engine.cpp

@@ -41,9 +41,8 @@ VmEngine<Policy, Oram>::create(
     m->blob_storage.assign(blob_data, blob_data + blob_size);
 
     // Validate blob via BlobView (pointing into owned storage)
-    auto blob_or = BlobView::create(m->blob_storage.data(), m->blob_storage.size());
-    if (!blob_or) return tl::make_unexpected(blob_or.error());
-    m->blob = *blob_or;
+    VMPILOT_TRY_ASSIGN(blob, BlobView::create(m->blob_storage.data(), m->blob_storage.size()));
+    m->blob = blob;
 
     // 2. Key derivation (doc 16 §6: all via BLAKE3_KEYED, not blake3_kdf)
     //
@@ -371,9 +370,7 @@ execute_one_instruction(VmExecution& exec, VmEpoch& epoch,
                         VmOramState& oram, const VmImmutable& imm) noexcept
 {
     // ── Phases A-C: FETCH + DECRYPT + DECODE ────────────────────────────
-    auto insn_or = pipeline::fetch_decrypt_decode(imm, exec, epoch);
-    if (!insn_or) return tl::make_unexpected(insn_or.error());
-    auto insn = *insn_or;
+    VMPILOT_TRY_ASSIGN(insn, pipeline::fetch_decrypt_decode(imm, exec, epoch));
 
     // ── Phase C': RESOLVE operands + FPE decode ─────────────────────────
     pipeline::resolve_operands(imm, exec, epoch, insn);
@@ -645,14 +642,12 @@ VmEngine<Policy, Oram>::dispatch_unit() noexcept
                         | (exec_.current_bb_id & m_id);
 
         // ── L.3: Always verify BB MAC ───────────────────────────────
-        auto mac_r = pipeline::verify_bb_mac(*imm_, exec_, *epoch_);
-        if (!mac_r) return tl::make_unexpected(mac_r.error());
+        VMPILOT_TRY(pipeline::verify_bb_mac(*imm_, exec_, *epoch_));
 
         // ── L.4: Always enter_basic_block ───────────────────────────
         exec_.branch_taken = false;
-        auto enter_r = pipeline::enter_basic_block(
-            exec_, *epoch_, *imm_, target);
-        if (!enter_r) return tl::make_unexpected(enter_r.error());
+        VMPILOT_TRY(pipeline::enter_basic_block(
+            exec_, *epoch_, *imm_, target));
 
         // ── L.5: RET_VM resume — branchless apply via MUX ───────────
         {
@@ -687,9 +682,8 @@ tl::expected<VmExecResult, DiagnosticCode>
 VmEngine<Policy, Oram>::execute() noexcept
 {
     while (true) {
-        auto r = dispatch_unit();
-        if (!r) return tl::make_unexpected(r.error());
-        if (*r == VmResult::Halted) {
+        VMPILOT_TRY_ASSIGN(r, dispatch_unit());
+        if (r == VmResult::Halted) {
             // Decode return value from register 0 using current FPE key.
             // After the last instruction's ratchet, regs[0] is encoded
             // under exec_.insn_fpe_key.  Convenience one-shot FPE_Decode.

runtime/src/vm_runner.cpp

@@ -54,12 +54,11 @@ template<typename Policy, typename Oram>
 tl::expected<VmExecResult, DiagnosticCode> VmRunner<Policy, Oram>::run() {
     auto blob = build_blob_internal();
 
-    auto engine = VmEngine<Policy, Oram>::create(
+    VMPILOT_TRY_ASSIGN(engine, VmEngine<Policy, Oram>::create(
         blob.data(), blob.size(), seed_,
-        load_base_delta_, init_regs_, num_init_regs_);
-    if (!engine) return tl::make_unexpected(engine.error());
+        load_base_delta_, init_regs_, num_init_regs_));
 
-    return engine->execute();
+    return engine.execute();
 }
 
 template<typename Policy, typename Oram>