fix(vm): preserve nested sessions in parallel block invocations (#17)

[Adityakk9031]

Summary

Addresses #17 by specifying and standardizing execution semantics for block invocations inside parallel contexts (parallel: or parallel for).

Context & Root Cause

When executing a named block containing sequential session: or call: statements (such as appending start and completion markers to an event ledger before and after work steps) inside a parallel: block or parallel for loop:

block process-item(item_id):
  session: ledger
    prompt: "Append 'phase-1-started' for {item_id}"
  let step1_result = session "Do step 1"
    prompt: "Process step 1 for {item_id}"
  session: ledger
    prompt: "Append 'phase-1-completed' for {item_id}"

Previous VM specifications did not define execution semantics for parallel block invocations. As observed by @MattKotsenas in #17, executing agents and VMs collapsed the block into a single natural-language directive for a subagent (e.g. "process item X through all phases"). This caused:

1. Collapsed nested sessions: The subagent executed everything in a single turn without dispatching discrete session: or call: child sessions.
2. Batched state / lost temporal ordering: Ledger appends and intermediate state updates were deferred and written simultaneously at the end with identical timestamps, eliminating observable phase duration and progress.
3. Unspecified branch isolation: Lack of an explicit convention for scoped execution frames during concurrent block runs.

Key Changes

1. VM Execution Specification (skills/open-prose/prose.md):
   • Added ### Block Invocation in Parallel Contexts under ### Parallel Execution.
   • Statement-by-statement execution: Mandates sequential statement evaluation for each parallel branch.
   • Session boundary preservation: Strictly forbids collapsing block statements into a single natural-language summary prompt; every session: / call: statement must be spawned as an independent, discrete session with its own prompt and completion barrier.
   • Sub-VM delegation protocol: When parallel branches are delegated to subagents, the coordinator must pass the unsummarized block body and instruct the worker to act as an OpenProse sub-VM executing statements sequentially.
   • Contemporaneous updates: Intermediate state, event ledger appends, and receipt updates must occur contemporaneously when each statement completes, guaranteeing accurate temporal ordering and distinct timestamps.
   • Scoped execution frames: Iterations execute within isolated execution scopes (workspace/{node}/__scope/{execution_id}/).
   • Cross-referenced from #### 4d. Parallel Execution.
2. Language Reference (skills/open-prose/prosescript.md):
   • Added ### Parallel Block Invocations under ## Parallel Blocks, defining invariants: no statement collapsing, discrete nested sessions, contemporaneous updates, and isolated scope.
   • Updated ## Blocks And 'do' to emphasize that parallel block invocation preserves statement sequences and nested session boundaries.
3. Authoring Guidance (skills/open-prose/guidance/authoring.md):
   • Added explicit anti-patterns under ## Anti-Patterns:
     • Collapsing block invocations in parallel contexts into a single natural-language subagent task instead of preserving discrete statement-by-statement session execution and scoped checkpoints.
     • Batching intermediate ledger writes or state updates at the end of a parallel block branch instead of committing them contemporaneously after each statement completes.
4. State & Filesystem Specification (skills/open-prose/state/filesystem.md):
   • Added workspace/{node}/__scope/{execution_id}/* to the canonical directory artifacts table.
   • Documented ## Scoped Execution Frames with per-branch isolation, scratch isolation, and contemporaneous progress progression.
5. Conformance Tests (tests/open-prose/primitives/parallel-blocks.test.ts):
   • Added 17 automated Vitest tests validating specification sections, invariants, anti-pattern documentation, filesystem layout, and a behavioral simulation demonstrating advancing timestamps and discrete sessions under compliant VM execution vs. identical timestamps in the collapsed anti-pattern.

Verification

• pnpm vitest run tests/open-prose/primitives/parallel-blocks.test.ts (17/17 passed)
• pnpm vitest run tests/open-prose/primitives/ (29/29 passed)
• pnpm vitest run tests/open-prose/state/ tests/open-prose/forme/ tests/open-prose/compiler/ tests/open-prose/concepts/ tests/open-prose/skill-meta/ tests/open-prose/tenets/ (127/127 passed)

Closes #17