fix(core): remove dead ACT-R decay path, disambiguate ADAPTIVE_DECAY naming (#346)

[cdeust]

Summary

decay_cycle.py's ACT-R base-level-activation block (compute_decay_updates,
compute_actr_base_level, actr_activation_to_heat, _compute_actr_decay)
had zero production callers, and its name (adaptive_decay=True) collides
with the live, published ADAPTIVE_DECAY mechanism that lives elsewhere.

Investigation before deleting

The maintainer flagged mid-task that dead code is more often mis-wired
than abandoned, and asked for git-history evidence before deleting. Traced
the full history of the block:

1. Wired at the original commit (b80a5998).
2. Replaced by an ACT-R model in the "zetetic rewrite" commit
   (10359edc, "faithful paper implementations") — still wired.
3. Deliberately unwired by the A3 migration (b9997157), per an
   explicit design decision:
   docs/program/phase-3-a3-migration-design.md §6
   "Decay cycle post-A3 — DELETE" instructs: "keep the pure-math helpers
   referenced by tests, but unwire from the consolidate handler" — because
   memory heat decay moved to the SQL-side lazy effective_heat()
   (pg_schema.py).
4. The design doc's planned test migration (§6 table:
   test_effective_heat_stage_coefficients etc.) was in fact completed,
   just under different file names:
   tests_py/infrastructure/test_pg_alpha_integral.py,
   test_pg_effective_stage_parity.py, test_pg_decay_clock_anchor.py
   cover the SQL-side equivalent of what test_decay_cycle.py asserted for
   the Python path.

This is a traced, documented supersession decision, not a missed
wiring — deletion is the correct outcome per coding-standards.md §9
("if it's built, it must be called").

Changes

• mcp_server/core/decay_cycle.py: deleted the ACT-R block, _compute_single_decay,
  and the now-dead-only helpers (_hours_since_access, _hours_since_creation,
  _ACT_R_DECAY_D, _ACT_R_NOISE_S, _MIN_LIFETIME_HOURS,
  _HIGH_IMPORTANCE_THRESHOLD). Kept compute_entity_decay (the only
  production-called function). New module docstring explicitly disambiguates
  the deleted ACT-R model from the live ADAPTIVE_DECAY mechanism
  (thermodynamics.py / pg_recall.py / memory_config.py, behind the
  LoCoMo v3 ablation ΔMRR −0.0163/ΔR@10 −0.020 figure) — closing the
  name-collision.
• tests_py/core/test_decay_cycle.py: removed the dead-path test classes,
  kept TestComputeEntityDecay.
• tests_py/benchmarks/benchmark_harness.py (boy-scout): removed
  benchmark_decay_with_emotional_resistance (a standalone __main__
  script, never pytest-collected, calling the now-deleted function) and its
  report section, renumbering the remaining sections to avoid introducing a
  fresh numbering gap.

Acceptance criteria (issue #346)

• grep -rn "compute_decay_updates\|compute_actr_base_level" mcp_server/ tests_py/ returns nothing.
• ADAPTIVE_DECAY now resolves to exactly one mechanism (thermodynamics/pg_recall/memory_config), documented at the point a reader is most likely to conflate it (decay_cycle.py's module docstring).
• No benchmark regression possible — the deleted code had zero production callers, so no live retrieval path changes. Not re-running the full LoCoMo/BEAM suite (machine under heavy shared load; the argument is structural, not measured: an uncalled function cannot change a benchmark result).

Test plan

• pytest tests_py/core/test_decay_cycle.py tests_py/handlers/consolidation/ — 100 passed
• pytest tests_py/benchmarks/ tests_py/infrastructure/test_pg_alpha_integral.py tests_py/infrastructure/test_pg_effective_stage_parity.py tests_py/infrastructure/test_pg_decay_clock_anchor.py tests_py/invariants/test_I2_canonical_writer.py — 71 passed
• Manual smoke of the standalone benchmark_harness.run_all_benchmarks() — runs clean, section numbering verified contiguous
• ruff check . / ruff format --check . — clean on the full tree
• Mutation testing (scripts/mutation_check.sh) — not yet run, machine load-gated by the maintainer; exact command below, ready on request:

  scripts/mutation_check.sh "tests_py/core/test_decay_cycle.py tests_py/handlers/consolidation/test_decay.py" mcp_server/core/decay_cycle.py
  

Closes #346

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