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).

Manifest.toml

@@ -2,7 +2,7 @@
 
 julia_version = "1.12.6"
 manifest_format = "2.0"
-project_hash = "a623ff56053e3a56c1799a1cb2080ec48d933b73"
+project_hash = "ed6bd1b772452682c906ce1236b89ccb1b0876fc"
 
 [[deps.ADTypes]]
 git-tree-sha1 = "d9aaef7c63466eee4de23b4d9dad03629df54bea"
@@ -309,7 +309,7 @@ weakdeps = ["HTTP"]
     HTTPExt = "HTTP"
 
 [[deps.FileServer]]
-deps = ["HTTP", "JLD2", "JSON3", "Logging", "Lux", "Optimisers", "Oxygen", "UUIDs", "Zygote"]
+deps = ["HTTP", "JLD2", "JSON3", "Languages", "Logging", "Lux", "Optimisers", "Oxygen", "Random", "UUIDs", "Zygote"]
 path = "."
 uuid = "b3f1c2d4-5e6a-4b7c-8d9e-0f1a2b3c4d5e"
 version = "0.1.0"