Add per-parent rejected-proposal memory (fixes #379)

[aranya-chatterjee]

Fixes #379

Adds per-parent memory of rejected reflective-mutation attempts so the same parent doesn't regenerate the identical failed mutation indefinitely.

Changes:

• GEPAState.rejected_proposals_by_parent: dict[ProgramIdx, list[RejectionRecord]] (schema v6, backward-compatible with existing run_dirs)
• Populated in _accept_reflective_proposal's reject branch (engine.py), keyed by parent program id
• Surfaced in the reflection prompt (reflective_mutation.py) under "Earlier attempted edits from this parent and what went wrong" -- purely informational, not a hard tabu list
• Bounded to last 3 rejections per parent to avoid context bloat on hot parents

Tests: tests/test_rejected_proposal_memory.py -- covers state persistence, backward-compat load from pre-fix run_dirs, prompt-threading behavior (no history on first attempt, history surfaces on repeat parent, bounded to 3). All passing, no regressions in existing suite.

[semanticdiff-com]

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Changed Files

File	Status
src/gepa/core/engine.py	35% smaller
src/gepa/proposer/reflective_mutation/reflective_mutation.py	4% smaller
src/gepa/core/state.py	0% smaller
tests/test_rejected_proposal_memory.py	0% smaller