feat(durable): add union reconciliation by session identity

[jonasotoaguilar]

🔗 Linked Issue

Related to #117

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🏷️ PR Type

What kind of change does this PR introduce?

• type:bug — Bug fix
• type:feature — New feature
• type:docs — Documentation only
• type:refactor — Code refactoring
• type:chore — Build, CI, or tooling
• type:breaking-change — Breaking change

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📝 Summary

feat(durable): add union reconciliation by session identity — part of durable per-session checkpoints chain (feat/durable-project-checkpoints) for #117. Union reconciliation by session identity (live session.list + durable checkpoints).

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📂 Changes

File / Area	What Changed
src/tokenmeter/durable/reconcile.ts	Union reconciliation by session ID
test/durable-reconcile.test.ts	Reconciliation tests

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Chain Context

Field	Value
Chain	feat/durable-project-checkpoints
Tracker PR	#118
Position	3 of 20
Base	feat/durable-merge
Depends on	#120 (feat/durable-merge)
Follow-up	#122 (feat/durable-checkpoints)
Review budget	362 / 400
Starts at	feat/durable-merge (#120)
Ends with	Union reconciliation by session identity (live session.list + durable checkpoints)

Chain Overview

main
 └── feat/durable-project-checkpoints (tracker, PR #118)
     └── feat/durable-platform-paths (#119)
          └── feat/durable-merge (#120)
               └── 📍 feat/durable-reconcile (#121)
                    └── feat/durable-checkpoints (#122)
                         └── feat/durable-migrate (#123)
                              └── feat/durable-deleted (#124)
                                   └── feat/durable-project-cutover (#125)
                                        └── feat/durable-project-db (#126)
                                             └── feat/durable-project-legacy-adapter (#138)
                                                  └── feat/durable-browser-detail (#127)
                                                       └── feat/durable-browser-tests (#128)
                                                            └── feat/durable-cost-fallback-1 (#129)
                                                                 └── feat/durable-cost-fallback-2 (#130)
                                                                      └── feat/durable-cost-fallback-3 (#131)
                                                                           └── feat/durable-harness-1 (#132)
                                                                                └── feat/durable-harness-migration (#136)
                                                                                     └── feat/durable-harness-2 (#133)
                                                                                          └── feat/durable-finalize-legacy-removal (#140)
                                                                                               └── feat/durable-docs (#134)
                                                                                                    └── fix/durable-project-observed-cost (#142)

Scope

• Includes: feat(durable): add union reconciliation by session identity
• Excludes: Next layer: feat(durable): add WAL checkpoint store with monotonic batch upsert

Autonomy

• CI is expected to pass for this PR branch
• This PR has one deliverable scope
• This PR can be rolled back without unrelated changes
• Tests, docs, or manual verification cover this unit

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🧪 Test Plan

bun run typecheck
bun test

• Type check passes (bun run typecheck — no errors; per-layer focused)
• Tests pass — per-layer focused durable tests green; stack-top integrated bun test 389/389, bun run typecheck, bun run build verified on fix/durable-project-observed-cost (fix(project): use observed message-derived cost for Project checkpoints #142, top of 20-PR stack)
• Build passes (bun run build — verified on top; per-layer build green where applicable)
• Manual canary pending (runtime live verification on final top — stack remains draft)

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🤖 Automated Checks

The following checks run automatically on this PR:

Check	Status	Description
Check PR Cognitive Load	⏳	PR should stay within 400 changed lines (additions + deletions) or use maintainer-applied size:exception
Check Issue Reference	⏳	Tracker/default PR: Closes/Fixes/Resolves #N; child PR: Related to #N
Check Issue Has status:approved	⏳	Linked issue must have been approved before work began
Check PR Has type:* Label	⏳	Exactly one type:* label must be applied

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✅ Contributor Checklist

• PR is linked to an issue with status:approved
• PR stays within 400 changed lines, or I have requested/obtained maintainer-applied size:exception with rationale documented
• Tracker/default PR uses Closes #N, child PR uses Related to #N
• If chained, this PR targets tracker/parent branch, not main
• I have added the appropriate type:* label to this PR
• Lint and format checks pass (check-only, no mutation)
• Tests pass (per-layer + stack-top integrated)
• I have updated documentation if necessary
• My commits follow Conventional Commits format
• My commits do not include Co-Authored-By trailers

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💬 Notes for Reviewers

Part of stacked delivery feat/durable-project-checkpoints (20 PRs). Review bottom→top against immediate parent (feat/durable-merge). This PR is draft until full chain review. Stack-top integrated verification: bun test 389/389, bun run typecheck, bun run build on fix/durable-project-observed-cost (#142). Manual live canary pending on final top.