docs(spec) add Phase 2 § Parallelism plan to build-spec

[jkeeley2073]

Summary

Add a ### Parallelism plan sub-section to docs/build-spec.md Phase 2, between § Non-goals and § Risks. Captures how the seven remaining Phase 2 scope items (after ADRs 0012 + 0013 shipped in PRs #65 + #66) execute in parallel where possible, given the established 2–3-active-PRs ceiling for one-developer-plus-AI work.

The plan articulates:

• Dependency core (sequential): 3 → 4 → 5. Item 3's seed enables item 4's write-back path; items 4 + 5 share OpdbSyncService and so must sequence to avoid merge pain.
• Independent surface (parallel-safe): items 6, 7, 8, 9 don't conflict with each other or with the core sequence.
• Three waves:
  • Wave 1 — items 9 (work-email denylist) + 7-round-1 (close deprecated Dependabot PRs) + 3 (seed ingestion_sources). Three PRs in parallel; no file conflicts.
  • Wave 2 — items 6 (Playwright bump) + 8 (Stern asymmetry route ii) + 4 (OPDB --dry-run code + operational hand-off) + 7-round-2 (clean Dependabot bumps sequenced).
  • Wave 3 — item 5 (OTel groundwork). Last because it shares OpdbSyncService with item 4 and benefits from item 4's real run as the first telemetry source to instrument.
• Sizing: ~8–10 PRs + 1 operational hand-off, matching the earlier 8–12 ballpark.
• Conventions: item 4's operational run is a phase-retrospective hand-off (not a PR); item 7 triage PRs get brief descriptions and skip /local-review; docs/observability.md (Wave 3) defines the OTel inventory pattern Phase 3 / 4 / 5 inherit.

The plan is forward-looking spec content; it belongs in the build-spec next to the scope items it sequences, not in session memory.

Test Plan

• Docs-only PR; no code paths affected.
• dotnet build PinballWizard.slnx --nologo and dotnet test PinballWizard.slnx are expected to remain green.
• Cross-references verified: pointers to guardrails.md § Per-PR gate and § Risk register resolve; pointers to § Exit criteria and § Scope items in this same Phase 2 section resolve.

Out of Scope

• Wave 1 execution. No actual scope items 3 / 7 / 9 land in this PR — that's the next set of PRs once this parallelism plan is merged.
• docs/observability.md — created in Wave 3 (item 5), not now.
• Any Phase 3 / 4 / 5 work. Phase 2 closes its 9 scope items first per build-spec.md.

Checklist

• CI is green (build + test + coverage + CodeQL + sanitization) — will verify after the workflows run
• PR title follows the Conventional Commits format
• If this is a new architectural decision, an ADR has been added — N/A; this is execution sequencing within an already-approved phase
• If user-visible behavior changes, README.md and/or docs/ are updated — yes, build-spec.md updated
• If a memory is now stale, it has been updated or removed — N/A; the session-handoff memory written at the start of this turn already references this plan
• No TODO / FIXME / commented-out code committed
• No new entries in <NoWarn> without justification — N/A; no code

Pre-push self-audit (additive PRs)

N/A for this PR — docs-only, no scraper / options class / extension / additive code surface. Per docs/guardrails.md § "Per-PR gate" and CLAUDE.md § "PR self-audit", "Doc-only PRs and pure dependency bumps may skip" the audit. Identity check still verified:

• git log -1 --format='%an <%ae>' → Jim Keeley <94459922+jkeeley2073@users.noreply.github.com> ✅

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