GFQL monolith shrinkdown S3: split lowering.py by concern boundaries

[lmeyerov]

Summary: Execution slice S3 under #1260.

Split lowering.py by concern into focused modules while preserving behavior and import compatibility.

Scope in:

• Move bounded reentry orchestration helpers into reentry-focused module(s)
• Separate projection-planning utilities from general lowering logic
• Keep stable call-sites via compatibility re-exports

Scope out:

• Feature/semantic changes
• CompiledCypher compatibility surface reduction (S4)

Acceptance:

1. lowering.py LOC decreases materially and delta is reported in Meta: GFQL monolith shrinkdown (lowering.py + CompiledCypher cleanup) #1260.
2. Concern boundaries are explicit and documented.
3. Existing imports keep working via re-exports.
4. Cypher/GFQL test suites and CI stay green.

Related: parent #1260, prior slice #1295, umbrella #989.

[lmeyerov]

Landed via PR #1303: https://github.com/graphistry/pygraphistry/pull/1303\n\nMerge commit: 8f0c509\nMerged: 2026-05-05\n\nS3 receipts:\n- Split concerns out of lowering.py into:\n - graphistry/compute/gfql/cypher/projection_planning.py\n - graphistry/compute/gfql/cypher/reentry/runtime.py\n- Kept compatibility call-sites via delegators/re-exports in lowering.py\n- Added targeted S3 delegator-forwarding amplification tests\n- CI + RTD green on merge