Replaces the Phase 0 stub. Reads PAM_USER, rejects usernames containing /,
\0, .., or that are . / .. / empty. stat()s /var/lib/authforge/pending/<user>:
present -> emits the design-doc user-facing message + PAM_AUTH_ERR; ENOENT
-> PAM_IGNORE (lets the rest of the stack decide); other errno -> logs and
fails closed (PAM_AUTH_ERR).
Builds clean against libpam0g-dev with -Wall -Wextra -Werror -fPIC -O2.
Resulting .so is a 16KB stripped-with-buildid x86-64 ELF; passes file(1) sanity.
pamtester smoke recipe lands in Task B2 alongside the test PAM stack file.
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