M4-PR3: implement PredicatePushdownPass rewrite using pass framework + safety primitives

[lmeyerov]

Parent: #1179
Depends on: #1180 (pass framework), #1181 (pushdown safety primitives)

Objective

Implement the actual PredicatePushdownPass logical-plan rewrite now that framework and safety primitives are merged.

Scope

• Add PredicatePushdownPass implementation under graphistry/compute/gfql/passes/.
• Rewrite Filter(input=PatternMatch(...)) by pushing safe predicates into PatternMatch.predicates.
• Respect OPTIONAL MATCH null-extension semantics via existing safety helpers in gfql/ir/pushdown_safety.py.
• Respect WITH/ScopeFrame barriers (no pushdown across scope-reset boundaries).
• Keep pass pure (no in-place mutation).

Acceptance

• Safe predicates are pushed into PatternMatch.predicates and residual predicates remain as Filter when only partially pushable.
• Null-rejecting predicates on optional-arm aliases are not pushed into optional arms.
• WITH-boundary barrier blocks pushdown where required.
• Pass executes through PassManager and verifier remains green after pass application.
• No semantic regression in existing Cypher OPTIONAL behavior tests.

Test Gate

• Unit tests for full-push, partial-push, and no-push cases.
• Negative tests proving optional-arm null-rejecting predicates stay above optional match.
• Boundary tests proving WITH/ScopeFrame blocks backward pushdown.
• Integration test proving logical plan flows through pass manager and produces stable output plan shape.

Non-goals

• No IndexRewritePass in this issue.
• No broad cost-based optimization work in this issue.
• No physical planner route changes in this issue.

Notes

• Prefer small deterministic rewrites and explicit plan-shape assertions in tests.
• If discovered edge cases require additional safety helper extensions, track those in this PR scope only when directly required for predicate pushdown correctness.

[lmeyerov]

Merged via #1188 and closed as completed.

Delivered in this lane:

• PredicatePushdownPass rewrite implementation
• pass-manager route integration
• focused unit/integration coverage including optional-arm safety hardening
• CI green at merge/update time

Meta completion tracked in #1179.