[FEAT]: Canonical versioned trace/result serializer (WS0-05 PR 1)

[Nina Chikanov (nina-msft)]

WS0-05 — PR 1: Canonical versioned trace/result serializer (Decision D6 gate)

First PR of the WS0-05 execution plan. Pure addition — no existing consumer is
rewired here, so nothing breaks; later PRs project reporting/xdist onto this module.

What this lands

• **
  ampart/core/serialization.py** — the single, neutral full-fidelity
  Result ⇄ dict round-trip. Public surface is ResultRecord.to_dict /
  from_dict plus thin serialize_result / deserialize_result wrappers.
• Single root version rampart.trace.v1 stamped on every record; decoding
  dispatches through a _DECODERS registry and fails closed on an unknown major.
  Independent of the xdist transport envelope version (rampart.xdist.v2).
• Value-domain-only canonical layer (design §5): enums → .value and fail
  closed on unknown; harm_category passthrough string; floats must be finite;
  free-form maps must be JSON-safe (no repr()/default=str fallback — that
  stays in the xdist transport). Reserved xdist bookkeeping keys are stripped from
  the canonical body.
• Binary payloads fail closed on encode and decode pending the WS7 artifact
  resolver, rather than being coerced to PayloadFormat.TEXT.
• Covers all 9 nested types including the new PopulationRef.
• Migration policy committed at docs/concepts/trace-schema.md (design §4):
  additive-optional = no bump; structural = major bump; missing = not recorded;
  named reserved additive slots so WS8 provenance needs no hard migration.

Tests

tests/unit/core/test_serialization.py — round-trip, field-exhaustiveness via
dataclasses.fields(), version dispatch + fail-closed, additive tolerance,
missing-field defaults, value-domain fail-closed, and binary-payload fail-closed.
18 pass; full core suite (398) green; ruff + ty clean.

Still to do in this PR (draft)

• Generate the open trace.v1.schema.json from encoder output over a fixtures
  corpus + a CI git diff --exit-code drift gate (design deliverable).
• Design sign-off (§7 checklist) recorded before merge — the entry gate.

Refs WS0-05 (#10594). Blocks WS6/WS7; shares the serializer with #10596 (xdist merge).

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