feat: hidden-active recovery benchmark, EF=4.0 at k=5, 75 tests - #3

* feat: elastase resistance + aggregation propensity scoring

Two new computational features to reduce wet-lab failure rate:

1. Elastase resistance (GAP #3 from audit):
   - physchem.py: ELASTASE_SITES = {A,V,S} (HNE primary P1 substrates);
     interior_protease_sites() reused; elastase_site_density and
     interior_elastase_sites added to compute_features() output.
   - stability.py: serum_stability_score() extended from 2-protease
     (trypsin/chymotrypsin) to 3-protease model. Weighted sum with
     trypsin:2 > chymotrypsin:1 > elastase:0.5. Helix-forming AMPs
     with high Ala content are now correctly penalised at infection
     sites where HNE is abundant (>1 µM). Denominator 3.5 = sum of
     weights; backward-compatible (missing elastase key → 0.0).
   - Literature: Bieth (1986); Doherty et al. (1991 Biochemistry).
   - 12 new tests in test_elastase_stability.py.

2. Aggregation propensity (GAP #1 from audit):
   - physchem.py: AGG_HYDROPHOBIC = {V,I,L,M,F,W}; new function
     aggregation_propensity() — two-component model:
       0.7 × interior_run_risk (run ≥ 4 → ramp 0→1 over 5 residues)
       + 0.3 × beta_branched_density_risk (V,I,T > 20% → ramp 0→1)
     Returns [0,1]; key added to compute_features() output.
   - synthesis.py: synthesis_feasibility_score() now includes:
       if agg > 0: score -= min(agg * 0.25, 0.20)
     Max penalty = 0.20 (capped). Backward-compat (missing key → 0).
   - Literature: Quittot et al. (2017 Protein Sci);
                 Wurth et al. (2006 J Mol Biol).
   - 22 new tests in test_aggregation_propensity.py.

Impact: AUROC=0.814 (unchanged; elastase/aggregation only affect
synthesis and stability, not the activity score that drives AUROC).
Total test count: 1122.

* fix: address code review HIGH issues for PR #49

- physchem.py: aggregation_propensity() now runs hydrophobic-run check
  on the FULL sequence (was interior-only), aligning with QC regex
  HYDROPHOBIC_RUN_RE which also scans the full sequence. Docstring claim
  "same threshold as QC HYDROPHOBIC_RUN_RE flag" is now factually true.
  Also: fixed saturation comment "run ≥ 9" → correct "run ≥ 8";
  removed redundant `if max_run >= 4` guard (max(0, ...) already handles it);
  added Ala limitation note to docstring (Ala aggregation not modelled).
- test_aggregation_propensity.py: replaced test_run_of_4_triggers_risk
  (which was passing for wrong reason — via beta_risk, not run_risk)
  with three tests: test_run_of_4_triggers_run_risk (asserts score > 0.14
  which guarantees the run component), test_run_of_4_boundary_exact_run_component
  (verifies exact math: KVLLLK → run=4 → run_risk=0.14), and
  test_run_of_8_saturates_at_max_run_risk (verifies saturation at run=8).
- Makefile: update test count to 1124.