UN-3636 [FIX] Stop re-running cross-tier deps in every tier leg

[chandrasekharan-zipstack]

What

Bound the rig's depends_on expansion to the requested tier when --tier is passed.

Why

integration-workflow-execution and e2e-smoke both declare depends_on: [unit-sdk1, unit-workers]. Because --tier expanded deps transitively with no tier bound, those two unit groups ran three times per CI run — once in the unit leg, again in the integration leg, again in the e2e leg.

Measured on run 29891886572 (main):

group	unit leg	integration leg	e2e leg
unit-sdk1	21s	27s	27s
unit-workers	37s	51s	51s

≈136s of duplicate execution per run. On the cloud CI runner (custom-k8s-runner, ~2 xdist workers) the same duplication costs ~350s.

Changes

• tests/rig/selection.py: with --tier, drop dep-expanded groups outside that tier. Explicitly named groups are never dropped.
• tests/rig/tests/test_selection.py: +3 tests — cross-tier dep not pulled in, explicit request survives the filter, expansion unchanged without a tier filter.

Safety

• Intra-tier deps still expand and topo-order (e2e-smoke → e2e-login).
• Gating is unaffected: blocked_by intersects with groups that failed in the same run, so a dep that wasn't run simply doesn't gate — and its own tier leg gates it.
• tests.rig validate OK — 18 groups, 16 critical paths. Full rig suite: 74 passed, 1 skipped.

🤖 Generated with Claude Code

https://claude.ai/code/session_01RnLaN45ShBbcCXWZkqCThz

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[chandrasekharan-zipstack]

Folding these changes into #2188 instead of carrying a second rig PR — the cross-tier dep fix now rides on feat/rig-extra-manifests (commit 793b32f).