Add stage-5 unknown-format discovery: header clustering + calibration

Implements phase A of the DESIGN_clustering.md design: a Dirichlet-process
mixture of per-position categoricals over the first 32 header bytes (257-symbol
alphabet) that clusters the binary/ pile by file format, plus signature
extraction and promotion nomination. All base-Julia (a Lanczos loggamma keeps
the Dirichlet-multinomial marginal dependency-free).

- src/cluster.jl: header_symbols feature extraction, collapsed Gibbs sampler
  (phase A), sequential CRP-predictive assignment (phase B core), signatures/
  promotion, and ARI/V-measure calibration metrics.
- bin/cluster_calibrate.jl: grid-tunes hyperparameters against magic-collapsed
  ground truth and cross-checks a model-free NCD (gzip) baseline.
- FS_CLUSTER_*/FS_PROMOTE_* config knobs; wire cluster.jl into the module.
- Tests for the three DESIGN §10 assertions plus the model primitives.

Calibrated defaults (n=32, alpha=1.0, beta=0.1) recover known formats at
ARI 0.77 (0.885 excl. tar); docx+zip and the ELF family merge correctly and the
NCD baseline agrees. DESIGN §11 records the results and three assumptions the
data corrected (tar/ELF header-zero merge, the cold-start seeding deadlock, and
the Bernoulli signature / Occam-penalized restart scoring).

Project.toml

@@ -12,6 +12,7 @@ Logging = "56ddb016-857b-54e1-b83d-db4d58db5568"
 Lux = "b2108857-7c20-44ae-9111-449ecde12c47"
 Optimisers = "3bd65402-5787-11e9-1adc-39752487f4e2"
 Oxygen = "df9a0d86-3283-4920-82dc-4555fc0d1d8b"
+Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 UUIDs = "cf7118a7-6976-5b1a-9a39-7adc72f591a4"
 Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 
@@ -24,6 +25,7 @@ Logging = "1.11.0"
 Lux = "1.31.4"
 Optimisers = "0.4.7"
 Oxygen = "1.10.2"
+Random = "1.11.0"
 UUIDs = "1.11.0"
 Zygote = "0.7.11"