fix(frontend): keep zonal EC2 AZ in purchase variant identity and cell key

[cristim]

Part of #792 (PR3). Backend identity landed in #807; this keeps the frontend consistent with it.

What

• samePurchaseVariantIdentity now compares details.availability_zone for EC2/compute, so the purchase modal can no longer switch term/payment to a variant in a different AZ (the backend returns 409 for such a purchase). Regional recs have no AZ on either side and still match.
• cellKey appends the AZ only when a rec has one, so zonal recs in different AZs are separate cells (and separate radio-selection groups) and every other cell key is unchanged.
• availability_zone? added to LocalRecommendation and the API Recommendation type.

Ordering

Must merge with or before PR2b (the store change that lets AZ-only variants coexist); until then the backend can still hold only one row per 8-tuple.

Verification

• jest (maxWorkers=2): full suite 3216 passed; tsc, lint (0 errors) and webpack build clean.
• New tests: two zonal recs differing only by AZ are excluded from each other's variants (ec2 and compute), a regional rec keeps matching its own variants, groupRecsByCell splits per AZ and leaves the regional key unchanged.
• Mutations: removing availability_zone from the identity fields fails 2 tests; removing the cellKey suffix fails 1 test.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 3fde4ef and e1f5924.

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📒 Files selected for processing (5)

• frontend/src/__tests__/purchase-modal-submit.test.ts
• frontend/src/__tests__/recommendations.test.ts
• frontend/src/api/types.ts
• frontend/src/recommendations.ts
• frontend/src/types.ts

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[cristim]

Independent gate review (Opus 5.5): APPROVE at e1f592440599be9939247364619c5b0cb184cceb

Scope reviewed: full diff (5 files, +34/-2) against origin/main; head contains main.

Findings: none blocking.

• samePurchaseVariantIdentity (ec2/compute) now compares details.availability_zone; key matches the lib ComputeDetails.AvailabilityZone json tag (availability_zone, pkg@9dae64b216d4 common/types.go:560). Regional recs: both sides undefined -> still equal, so regional variant narrowing is unchanged.
• cellKey appends the top-level availability_zone only when non-empty; field name matches RecommendationRecord.AvailabilityZone json tag on main (internal/config/types.go:539). Recs reach state via object spread (app.ts:447/595), so the field survives. All cellKey consumers (grouping, radio enforcement, pickBestVariantPerCell, loadedCellVariants, expand set) get per-AZ cells; non-zonal keys are byte-identical. Other composite keys (sp-group, bulk bucket at recommendations.ts:4151) are not per-resource identity and need no AZ.
• Tests: two zonal recs differing only by AZ (ec2 + compute) in the modal, zonal cell split, and a regional EC2 rec regression case.

Local verification (macOS, git archive of the exact SHA, under cc-heavy-run lock, nice 15):

• npx jest --maxWorkers=2: 102/102 suites, 3216 passed, 1 skipped, exit 0
• npm run lint 0, npx tsc --noEmit 0, npm run build 0
• Fails-before: with main's recommendations.ts and the PR tests, 3 of the 4 new tests fail (2 AZ modal cases, cell split); the regional regression case passes before and after, as intended.
  Evidence type: jest/jsdom fixtures exercising the real modal and grouping code; no real-account verification (frontend-only, backend 409 guard from fix(purchase): make zonal EC2 AZ part of the guard, identity key, idempotency key and rec ID #807 is the server-side backstop).

CI at this SHA: all required checks green (CI Success, Run pre-commit hooks) plus Unit/Integration/E2E/Lint/Build/Security. Only failure is Playwright ladder-config.spec.ts (Dismiss-notification strict-mode), the known main failure #802, not required; this is why mergeStateStatus is UNSTABLE rather than CLEAN.
Mutation sweep: not applicable (frontend change, per owner policy 2026-10-12).