spec: ReversibilityDistance contract + Appendix-A reference estimator (E13/WS-D)

[mdheller]

Epoch E13 / workstream WS-D — reversibility distance as a civic primitive

Lands ReversibilityDistance, the published civic meter of the reidentification economy
(reidentification-economy Appendix A), plus a self-proving reference estimator.

To single one subject out of a population N costs log2 N bits (the singling-out budget).
The reversibility distance D = E[log2 k] is the residual identity entropy — the bits an
adversary must still acquire to reidentify. D == 0 means fully reversible. D is the same
number as a MultiversealTwin's ε-unlinkability budget (ADR-0014): privacy, capacity and
reidentifiability are one ledger, with two orthogonal controls — a DP rate-limit on ΔD, and a
capture receipt gating whether the identifying query Q is produced at all.

Files added

• schemas/ReversibilityDistance.json — JSON Schema 2020-12, $id .../v2/ReversibilityDistance.json, additionalProperties:false, URN prefix urn:srcos:reversibility-distance:.
• examples/reversibility_distance.json — canonical example.
• tools/validate_reversibility_distance_examples.py — schema + invariants (dBits>=0; reidentified == (dBits==0); unicity-curve well-formed; budgetBits ≈ log2 N).
• tools/reversibility_distance.py — numpy-only Appendix-A estimator (unicity_closed_form, reversibility_distance, fit_unicity_exponent). Its __main__ self-test reproduces the appendix's worked examples (Sweeney N=2.48e8 → ε≈0.921; de Montjoye N=1.5e6 → r≈73.5, coarsened p=4→ε≈0.44, p=5→ε≈0.978) with asserts.
• docs/adr/0020-reversibility-distance-meter.md — ADR (relates to ADR-0014).
• Makefile — wired validate-reversibility-distance-examples into the validate: aggregate.

Verification (all pass)

• Estimator self-test: all asserts OK, reproduces Appendix A, exit 0.
• validate_reversibility_distance_examples.py: OK (schema + invariants).
• scripts/check_duplicate_schema_ids.py: OK: 353 unique schema $id values.
• tools/validate_schema_references.py: all $refs resolve.

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