docs(adr-0002): restore backend-consumer framing for asap-precompute-rs

[zzylol]

Summary

Surgical doc-only fix to ADR-0002. PR #242 over-deleted text when
fixing edge/backend terminology — I parsed the user's clarification
("backend's precompute engine is separate design") as "backend
doesn't depend on asap-precompute-rs at all", which was wrong.

The correct scoping is:

• Backend QUERY-side engine (PromQL aggregation, storage, query
  planning) is a separate design, NOT shared with the edge runtime.
• Backend INGEST path (envelope parsing, delta apply, sketch
  state reconstruction, merge) IS shared with the edge runtime —
  this is exactly what asap-precompute-rs provides.

Edits

• §Context: list the backend ingest path back as in-scope, while
  scoping out the backend's query-side engine.
• §What gets extracted → asap-precompute-rs bullet: restore
  dual-purpose framing — consumed by (a) future Rust-based edge
  agents (Vector, OTAP-Rust receiver), AND (b) ASAPQuery-backend's
  ingest path; backend's query-side engine kept separate.
• §Positive consequences: restore the agent / backend dedup
  bullet — explicit that the SHARED ingest pieces (envelope parsing,
  delta apply, sketch reconstruction, merge) are deduplicated, NOT
  the backend's query-side engine.
• §Performance contract: split the Rust bullet into
  edge-side (mirrors asap-precompute-go bit-identically) and
  backend-ingest-side (bit-identical envelope parsing across
  languages — bytes from sketchlib-go must reconstruct via
  asap_sketchlib). Reference issue asap_sketchlib ↔ sketchlib-go byte-format harmonization (cross-language parity) #243 as the cross-language
  byte-alignment prerequisite for backend integration (Phase 3
  step 3).

Test plan

• Doc-only diff (docs/adr/adr-0002-extract-precompute-runtime.md,
  +45/-18). No code touched.
• No submodule pointer drift.
• No residual "edge-only" / "backend doesn't depend" phrasing
  remains (grep clean).
• Reviewer confirms the new framing matches the agreed scoping
  (ingest = shared, query = separate).

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