[IR-P6][WS5] Phase 6 Sessionized runtime state (epic)

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Problem statement

Phase 6 needs a controlling epic to coordinate deliverables, acceptance criteria, and traceability for Sessionized runtime state.

Intended invariant

Replace global mutable registries in hot paths with explicit session/engine scoped state.

Scope boundaries

• Coordinates Phase 6 roadmap scope only
• Tracks delivery issues and phase exit gate evidence
• Defers unrelated feature expansion outside IR roadmap scope

Deliverable checklist

• Sub-issues for all phase deliverables are opened and linked
• Phase deliverables are implemented and verified
• Exit gate evidence is linked in roadmap and issues
• Migration and docs impact fields are kept synchronized

Validation plan

Phase 6 closes only when linked sub-issues are complete and roadmap phase exit gates are satisfied with evidence.

Rollback/failure mode

If scope expands or deliverables split, create follow-up issues and update roadmap + issue links in the same work session.

Roadmap reference

Roadmap reference: docs/plans/2026-06-19-001-ir-first-roadmap.md (Phase 6)

Migration impact

minor - phase-level impact estimate; refine on each sub-issue.

Documentation impact

update-existing - phase requires synchronized docs updates with implementation.

Sync status

Synced with roadmap: 2026-06-19

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Phase sub-issues:

• [IR-P6][WS5] Add Session/UnitOfWork abstraction #97 Add Session/UnitOfWork abstraction
• [IR-P6][WS5] Replace global registries in core paths #98 Replace global registries in core paths
• [IR-P6][WS5] Ship temporary compatibility shim #99 Ship temporary compatibility shim

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Refresh sync update: reviewed against current roadmap and Phase 7/8 sequencing (v0.13.0 cutover). No scope change required for this issue at this time.

Roadmap reference: docs/plans/2026-06-19-001-ir-first-roadmap.md.