feat(coord-fed): Phase 1.2 — Zig coord_identity.zig + 4 FFI exports

ADR-0016 Phase 1.2. Realises the C-ABI contract documented in
LocalCoord.Identity:

  boj_coord_identity_init(key_path)           — generate-or-load seed,
                                                persist at 0600
  boj_coord_identity_get_pubkey(out, out_len) — 32B pubkey export
  boj_coord_identity_load_known_peers(toml)   — load trust list
  boj_coord_identity_known_peer_count()       — count of loaded peers

* coord_identity.zig new file alongside coord_durability.zig — keeps
  the identity surface isolated from the main coord state machine.
  Singleton process state guarded by a mutex; no allocator threading
  needed for Phase 1's fixed-size known-peers array (cap = 64).

* std.crypto.sign.Ed25519.KeyPair.generateDeterministic for seed →
  keypair derivation (Zig 0.15.x API). Honest error propagation on
  the (essentially-impossible) IdentityElementError path.

* Minimal hand-rolled TOML-shaped parser for known_peers.toml — no
  third-party dep, parses [[peer]] blocks with id/pubkey/host/port
  fields. Unknown keys silently ignored (forward-compat). Missing
  file is 0 entries, not an error, so the bus starts cleanly on
  first run before any trust has been established.

* local_coord_ffi.zig adds `comptime { _ = @import("coord_identity.zig") }`
  so the new pub-export-fn symbols are linked into the shared library
  without a circular dependency or restructure of the main FFI file.

* Zig tests in-file (FFI roundtrip, hex decode positive/negative,
  TOML single/multi-block, missing-field rejection, missing-file
  load_known_peers). `zig build test` picks them up via the module
  import; no build.zig change needed.

NOT compiled locally — zig is devShell-only in this dev environment
(no `nix` either). API choices kept conservative for Zig 0.15.x
(splitScalar over splitAny, .public_key.bytes field access over
.toBytes()). CI's e2e workflow installs Zig 0.15.1 and will catch any
mismatch.

Author: claude