§H.12 step 18: zuse_eligibility.c -- read/add/membership-check

is_member() (fail-closed) and add() (idempotent, chains new devblocks
onto the tail as needed) over the item-17 eligibility-list devblock
chain, mirroring capsule_zuse_boot.c's magic/version/CRC-64 validation
convention. No callers yet.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QgooKd5hJNtTYqB6CyK5f9

FABRIC-3.md

@@ -4111,7 +4111,19 @@ work, not new invention.
   confirms the header is syntactically clean (verified via a standalone `gcc -fsyntax-only`
   compile, and via the struct-size static assert) and doesn't break the build. Verified
   3-arch boot to `ok>` (amd64/aarch64/riscv64).
-- [ ] **18.** Implement read/add/membership-check functions in C.
+- [x] **18. DONE 2026-09-03.** `include/starkernel/zuse_eligibility.h` +
+  `src/starkernel/capsule/zuse_eligibility.c`: `zuse_eligibility_is_member()`/
+  `zuse_eligibility_add()` over the item-17 devblock chain, mirroring
+  `capsule_zuse_boot.c`'s own magic/version/CRC-64 validation convention exactly (the fence
+  region is never zeroed at format time, so blank and corrupt devblocks are indistinguishable
+  and handled identically — merged into a single "absent" read failure). `is_member()` is
+  fail-closed (any read/chain error reads as "not eligible," never permissive). `add()` is
+  idempotent (re-adding an existing entry is a no-op success) and chains a fresh devblock onto
+  the tail when the current one fills, relying on `blk_meta_zone_write()`'s own existing
+  bounds check against `meta_fence_blocks` to surface an exhausted fence budget as an ordinary
+  write failure rather than duplicating that check here. No callers yet (that's item 19).
+  Verified 3-arch boot to `ok>` (amd64/aarch64/riscv64), each run in the foreground per
+  CLAUDE.md's QEMU rule.
 - [ ] **19.** Add a Zuse-only FORTH word to add an entry, gated by `zuse_session`.
 
 **Phase 7 — Message card gate + `ELEVATE-REQUEST` (H.8)**