[0.44 internal alpha.2] Replace universal signal invalidation with exact dependency subscriptions

[SisyphusZheng]

Part of #1155. Target: internal alpha.2 workspace.

Objective

Keep SignalEngine genuinely replaceable while making Signal -> Part/Region the only reactive DOM path. Preact Signals Core remains the default; engine selection is static per build and adds no per-update dispatcher.

Deliverables

• Freeze the minimal SignalEngine contract used by compiled OpenElement programs
• Keep the Preact Signals Core adapter as the default implementation
• Add a deliberately small alternate test engine and shared conformance suite
• Compile signal dependencies directly into Part/Region subscriptions
• Define build-time engine selection and diagnostics

Acceptance

• Preact and the alternate test engine pass identical ordering, batching, computed, effect and cleanup tests
• Engine selection changes no Part Program semantics
• No per-update virtual dispatch or second reactive DOM system exists
• Disconnect/reconnect and Region removal dispose subscriptions exactly once

Integration inputs

#1164, #1165, #1166

These inputs do not block workspace start. Use the frozen Alpha collaboration contract and owned fixtures; alpha.8 resolves the final integration.

Workspace boundary

Write only the paths assigned to this internal workspace in tools/config/v044-alpha-workspaces.json.

Authority

• docs/adr/ADR-0143-0-44-compiled-element-model-reentry.md
• docs/current/VERSION_PLAN.md

[SisyphusZheng]

Alpha.2 accepted evidence for SignalEngine:

• Base 6873f58; branch codex/v044-alpha.2-runtime-signals; head 5382ac5.
• Commit: 5382ac5 (feat(element): implement alpha.2 compiled runtime signals).
• RED: the focused fixed-Part validator test exited 1 before implementation with "[compiled-program] invalid Part Program spike: parts[0] has unknown part kind".
• GREEN: final focused compiled suite passed 23 tests (11 steps), package element tests passed 306 tests, and typecheck/lint/fmt/diff checks passed.
• Coverage: default and alternate protocol-only engines cover direct subscription, computed/effect ordering, lazy delivery, batching, and cleanup; runtime subscriptions remain local to each Part/Region.
• Push: exact deno task autoflow:push passed fmt:check, lint, typecheck, graph:check; git push origin codex/v044-alpha.2-runtime-signals created the remote branch.
• External handoff: per alpha.5, candidate source SHA 2714e7c has a mandatory autoflow:push failure at unowned packages/element/src/open-element-implementation.ts:56, which imports deleted open-element-hydration.ts. Alpha.2 did not edit alpha.5 or that unowned path and added no compatibility shim.

[SisyphusZheng]

Alpha.2 re-audit evidence at exact pushed head : alternate SignalEngine direct subscriptions now coalesce within a batch while preserving lazy initial delivery, computed ordering, and cleanup. Focused signal/runtime suite is green: 22 passed, 0 failed; signal protocol boundary check passed. Remote branch ref matches local HEAD exactly. Internal alpha only.