Allow plain and device-tagged listens to share a sockaddr (v0.7.1)

The previous wrapper skipped nginx's duplicate-listen check only
for listens that carried device=, so a `listen 80;` next to a
`listen 80 device=eth0 ...;` in the same server block was
rejected at config time. Under SO_BINDTODEVICE that restriction
tracked a real kernel constraint (device-tagged listens created
separate sockets, a bare listen alongside them was genuinely
ambiguous). Under the IP_PKTINFO model introduced in 450391a
the constraint no longer exists — all same-sockaddr listens
collapse to one wildcard kernel socket and attribution is a
per-connection cmsg readback — but the wrapper kept enforcing
the old rule by accident.

Extend the (cscf, sockaddr) dedup in the listen wrapper to
cover plain listens too: the first occurrence at a given
(server, sockaddr) pair calls nginx's handler and registers the
kernel socket, and every subsequent sibling — plain or
device-tagged — is accepted without tripping nginx's
duplicate-listen check. Device-tagged siblings additionally
push a binding into the attribution table as before; plain
siblings contribute only the seen-list entry. No code path
exercised by the existing 22 e2e tests changes behavior.

Update FR-1.5, the user-guide "shared port" section, the
module's top-of-function comments, and the test nginx.conf
comment to describe the relaxed rule. Bump VERSION and add a
debian/changelog entry for 0.7.1.

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

docs/design.md

@@ -84,10 +84,12 @@ Each requirement carries a unique identifier (`FR-X.Y` or `NFR-X.Y`) so that lat
 - **FR-1.3** A listening socket with neither `device=` nor `ipng_source_tag=` MUST be tagged with the configured default source string (see
   `ipng_stats_default_source`, FR-5.3). The default default is the literal string `direct`.
 - **FR-1.4** A listening socket with `device=X` but no `ipng_source_tag=` MUST be tagged with the interface name `X`.
-- **FR-1.5** Two `listen` directives that share `address:port` but differ in `device=` MUST coexist. Since no `SO_BINDTODEVICE`
-  is applied, the kernel delivers all matching SYNs to a single wildcard listening socket and the module distinguishes them by
-  reading `ifindex` from the per-connection cmsg — so "multiple device-tagged listens at the same port" at the config level
-  collapses to one kernel socket at runtime without any userspace contortions.
+- **FR-1.5** Two or more `listen` directives sharing `address:port` MUST coexist regardless of whether each carries `device=`.
+  Since no `SO_BINDTODEVICE` is applied, the kernel delivers all matching SYNs to a single wildcard listening socket and the
+  module distinguishes them by reading `ifindex` from the per-connection cmsg. The listen wrapper therefore deduplicates on
+  `(server block, sockaddr)` across both plain and device-tagged listens: the first occurrence registers the kernel socket,
+  and subsequent same-sockaddr siblings (plain or device-tagged) are accepted without tripping nginx's duplicate-listen check.
+  Device-tagged siblings additionally register an entry in the attribution table.
 - **FR-1.6** A `listen` directive that uses a wildcard address (`80`, `[::]:80`) together with `device=<ifname>` MUST attribute
   every connection whose ingress interface is `<ifname>` — regardless of which local address the client addressed — to that
   listen's source tag. Traffic on other interfaces MUST fall back to the configured default source (see FR-1.3).