v0.16.6

Move the sigil-lib and sigil-test dev-dependency pins from ^0.17 to ^0.19.

Same defect and same cure as sigil-crypto v0.16.5, reproduced independently on a different C surface:

  ^0.17   libsigil-tls.a builds (164998 B), tests FAIL
          "unsupported bytecode version 11 (expected 10)"
  ^0.19   libsigil-tls.a builds (165542 B), tests 19/19 PASS

Two packages, two unrelated native surfaces, one verdict: the pin does not protect the native build, and it is what breaks sigil test on a 0.19 host.

Patch rather than minor for the same reason as sigil-crypto: it raises a toolchain floor, and every consumer it would break is already broken.

sigil-tls and sigil-crypto are the only two extracted packages that declare sigil-lib, so between them they are the origin of every stale sigil-lib ^0.17 row in the estate. The wider sweep (193 stale manifest lines across 52 repos) is tracked separately and must follow these two rather than precede them.

Caveat recorded rather than omitted: sigil-tls's 19 passing tests do not carry their own sabotage arm. The positive control for this change was taken in sigil-crypto, where a 97/3 split was attributed to exactly the sha256 cases.