Add layered formal-verification harness

Adds four complementary layers to verify correctness, all runnable locally,
weakest-but-broadest to strongest-but-narrowest:

  0. Paper proofs (docs/verification.md): Hoare invariants, termination
     measures, and permutation arguments for every algorithm.
  1. Property tests (sort/property_test.go): testing/quick asserting ordering
     AND permutation for every sort. Closes a real gap -- the existing tests
     only checked .Sorted(), so a sort dropping/duplicating elements passed.
  2. make verify: go vet + staticcheck + go test -race -short, with -short
     gating of the large sizes in sort/search tests so the race build is quick.
  3. make verify-model: TLA+/TLC model check of sleep sort (termination,
     deadlock-freedom, sorted permutation) -- formal/tla/.
  4. make verify-formal: Gobra deductive proof (Viper+Z3) that a monomorphized
     insertion sort is memory-safe and sorted for all inputs -- formal/.

The static layer already found a latent bug: hash() used key<<10 on a generic
integer, which silently yields 0 for narrow key types (int8), degrading the
hash. Tests missed it because they only use int keys. Fixed by mixing in int64;
documented extensively in docs/case-study-hash-shift-bug.md.

Also cleans up dead code and a blank-identifier range flagged by staticcheck.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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