feat(element): implement MemoryDataAdapter per ADR-0095

[SisyphusZheng]

Problem

ADR-0095 (Accepted, 2026-06-10) explicitly promises:

Provide a single baseline implementation: MemoryDataAdapter — zero I/O, backed by Map.

The DataAdapter<T> interface exists at packages/element/src/internal/protocol/data.ts:10, but no implementation exists anywhere in the repository (zero grep matches for MemoryDataAdapter or implements DataAdapter).

This is a documentation-code drift: the ADR is Accepted but its deliverable is missing.

Evidence

• packages/element/src/internal/protocol/data.ts:10 — interface defined ✅
• grep -r "MemoryDataAdapter" packages/ — 0 matches ❌
• grep -r "implements DataAdapter" packages/ — 0 matches ❌
• ADR-0095 §Decision: "Provide a single baseline implementation: MemoryDataAdapter"

Fix

Option A (preferred): Implement MemoryDataAdapter (~20 lines):

export class MemoryDataAdapter<T = unknown> implements DataAdapter<T> {
  readonly name = 'memory';
  #store = new Map<string, T>();

  async get(key: string): Promise<T | undefined> {
    return this.#store.get(key);
  }

  async keys(): Promise<string[]> {
    return [...this.#store.keys()];
  }

  set(key: string, value: T): void {
    this.#store.set(key, value);
  }
}

Export from @openelement/element root (or build-utils if build-only).

Option B: Update ADR-0095 status to "Deferred" with rationale and target version.

Acceptance

• MemoryDataAdapter implemented with unit tests, OR
• ADR-0095 status updated to Deferred with explicit target version