docs: handover artefact for mdBook parity closure

[AlexMikhalev]

Adds .agent/handoffs/2026-08-12-mdbook-parity.md, the continuity artefact for the mdBook parity work released as 0.2.0.

Docs only — no code, no template, no config change.

Contents: verified state at fdc4625 (CI green, 174 tests, 87.1% coverage, live site at 575 pages), the artefact index across research/design/spec/verification/validation, a concrete resume procedure, and the open items (D-007 UBS external, D-008 accepted, mathjax-support parsed-but-inert, {{#include}} deliberately unmet).

The "Notes for the Next Session" section records the six findings that cost real time this cycle — Tera autoescape depending on registered template names, the server.rs route order that made live reload unreachable, the containment anchor, the .index-container layout constraint, and why three validation defects trace to Phase 2 design rather than implementation.

🤖 Generated with Claude Code

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📊 Performance Benchmark Results

Benchmark	Time
pagefind_init/pages/50	1.121 ms
pagefind_init/pages/100	2.127 ms
pagefind_init/pages/500	12.173 ms

Raw Results

{
  "timestamp": "2026-08-12T11:43:25+00:00",
  "benchmarks": [
    {
      "benchmark_name": "pagefind_init/pages/50",
      "mean": {
        "estimate": 1121400
      },
      "unit": "ns"
    },
    {
      "benchmark_name": "pagefind_init/pages/100",
      "mean": {
        "estimate": 2127200
      },
      "unit": "ns"
    },
    {
      "benchmark_name": "pagefind_init/pages/500",
      "mean": {
        "estimate": 12173000
      },
      "unit": "ns"
    }
  ]
}