docs: seed docs/proof-debt.md per trusted-base policy (#161)

## Summary
- Adds `docs/proof-debt.md` — schema-conformant index for the
trusted-base reduction policy
([standards#203](hyperpolymath/standards#203)).
- boj-server is the **reference implementation** cited in standards#203.

## Sections
| § | Content |
|---|---|
| (a) DISCHARGED | None — 5 class-J axioms unavoidable in Idris2 0.8.0.
|
| (b) BUDGETED | Reference to `docs/backend-assurance/` (BEAM-side
validation). |
| (c) NECESSARY | Table of all 5 axioms in
`src/abi/Boj/SafetyLemmas.idr`. |
| (d) DEBT       | None in ABI layer. |

## What this PR does NOT duplicate
Substantive content lives in `PROOF-NEEDS.md` +
`docs/backend-assurance/*`. This file references them.

## Companion
- standards#195, #203

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

docs/proof-debt.md

@@ -0,0 +1,107 @@
+<!--
+SPDX-License-Identifier: MPL-2.0
+SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell (hyperpolymath)
+-->
+
+# Proof Debt — boj-server
+
+**Schema**: [hyperpolymath/standards `TRUSTED-BASE-REDUCTION-POLICY.adoc`](https://github.com/hyperpolymath/standards/blob/main/docs/TRUSTED-BASE-REDUCTION-POLICY.adoc) (standards#203).
+
+boj-server is the **reference implementation** for the estate trusted-base
+reduction policy (cited as such in standards#203). The 5 class-J axioms
+this repo isolates in `src/abi/Boj/SafetyLemmas.idr` are the canonical
+example of disposition §(c) NECESSARY AXIOM, and the
+`docs/backend-assurance/` per-axiom files are the canonical example of
+external validation under §(b)-style discipline.
+
+This `docs/proof-debt.md` is a thin schema-aligned index. The substantive
+content lives in:
+
+- **[`PROOF-NEEDS.md`](../PROOF-NEEDS.md)** (repo root) — full audit table
+  with type signatures, classification rationale, and external-validation
+  pointers.
+- **[`docs/backend-assurance/`](./backend-assurance/)** — per-primitive
+  property tests + BEAM-side validation evidence
+  (`prim__eqChar.md`, `prim__strToCharList.md`, `prim__strAppend.md`,
+  `prim__strSubstr.md`).
+
+## (a) DISCHARGED in this repo
+
+*(None — the 5 class-J axioms are genuinely unavoidable in Idris2 0.8.0.
+Items would move here if a future Idris2 version exposes in-language
+soundness principles for `Char` / `String` primitives.)*
+
+## (b) BUDGETED — tested with a refutation budget
+
+The 5 axioms below are documented under §(c) (NECESSARY) rather than §(b)
+(BUDGETED) because they cannot be discharged in-language. **However**,
+each one is *externally validated* via the
+[backend-assurance harness](./backend-assurance/) — property tests
+running against the real BEAM runtime, exercising the Erlang/Elixir
+implementations of the underlying primitive operations.
+
+This dual-discipline (axiom-in-Idris2 + property-tested-externally) is
+the reference pattern the estate trusted-base policy recommends for
+extraction-boundary code.
+
+## (c) NECESSARY AXIOM
+
+All 5 are class-J ("genuinely unavoidable") per
+[PROOF-NEEDS.md §Axiom Audit](../PROOF-NEEDS.md) (2026-05-18). Each
+reduces to the same root cause: Idris2 0.8.0 treats `Char` and `String`
+as opaque primitive types whose operations are foreign functions with no
+constructors and no induction principle.
+
+| # | Site | Signature | External validation |
+|---|------|-----------|---------------------|
+| 1 | `src/abi/Boj/SafetyLemmas.idr:60` | `charEqSound : (c1,c2 : Char) -> c1 == c2 = True -> c1 = c2` | [`docs/backend-assurance/prim__eqChar.md`](./backend-assurance/prim__eqChar.md) |
+| 2 | `src/abi/Boj/SafetyLemmas.idr:67` | `charEqSym : (x,y : Char) -> (x == y) = (y == x)` | [`docs/backend-assurance/prim__eqChar.md`](./backend-assurance/prim__eqChar.md) |
+| 3 | `src/abi/Boj/SafetyLemmas.idr:218` | `unpackLength : length (unpack s) = length s` | [`docs/backend-assurance/prim__strToCharList.md`](./backend-assurance/prim__strToCharList.md) |
+| 4 | `src/abi/Boj/SafetyLemmas.idr:226` | `appendLengthSum : length (s ++ t) = length s + length t` | [`docs/backend-assurance/prim__strAppend.md`](./backend-assurance/prim__strAppend.md) |
+| 5 | `src/abi/Boj/SafetyLemmas.idr:233` | `substrLengthBound : LTE (length (substr start len s)) len` | [`docs/backend-assurance/prim__strSubstr.md`](./backend-assurance/prim__strSubstr.md) |
+
+Citation: see standards#203 §"Precedent" — boj-server's harness is the
+reference implementation the estate trusted-base policy directs other
+proof-bearing repos to adopt.
+
+## (d) DEBT — actively to be closed
+
+*(None in the Idris2 ABI layer. The 5 class-J axioms above are §(c),
+not §(d); they are not "to be closed" — they are load-bearing
+assumptions about the trusted base.)*
+
+Future work that would generate §(d) entries:
+
+- Migration to a future Idris2 version exposing in-language soundness
+  principles for primitive types — would let entries #1–#5 move from
+  §(c) → §(a) via discharge.
+- New `believe_me` introduced in non-SafetyLemmas.idr files — these
+  must be classified as §(a)/§(b)/§(c) before merge or land here as
+  §(d) with a deadline.
+
+## How to update this file
+
+When `src/abi/Boj/SafetyLemmas.idr` changes:
+
+1. Cross-check against `PROOF-NEEDS.md` — keep the per-site row counts
+   in sync.
+2. If a new `believe_me` is introduced anywhere outside that module,
+   add an entry here under §(d) or §(b) with an audit table entry in
+   `PROOF-NEEDS.md`.
+3. The future `scripts/check-trusted-base.sh`
+   ([standards trusted-base policy](https://github.com/hyperpolymath/standards/blob/main/docs/TRUSTED-BASE-REDUCTION-POLICY.adoc))
+   greps for naked `believe_me` outside `SafetyLemmas.idr` and fails CI
+   if it's not annotated or enumerated here.
+
+## Companion documents
+
+- [`PROOF-NEEDS.md`](../PROOF-NEEDS.md) — substantive audit table.
+- [`docs/backend-assurance/`](./backend-assurance/) — external-validation evidence.
+- [standards#195](https://github.com/hyperpolymath/standards/pull/195) — estate proof-debt audit.
+- [standards#203](https://github.com/hyperpolymath/standards/pull/203) — trusted-base reduction policy (the schema this file follows).
+- Memory: `project_boj_server_backend_assurance_harness.md` — long-term reduction plan (~3 months, ~1 PR per primitive).
+
+---
+
+🤖 Schema-conformant index seeded by Claude Code, 2026-05-26. boj-server is the
+reference implementation the estate trusted-base policy cites.