Fix shared-listen-include pattern across multiple server blocks

v0.3.0's listen wrapper treated every listen beyond the first at a
given sockaddr as a skip-core "duplicate", which was correct for
two `listen 80 device=X/Y;` lines in one server block but broke on
the real deployment pattern where every server-*.conf pulls in the
same `include listens.conf;`. Symptoms:

  * every server block after the first ended up with no listen
    directive processed, so nginx assigned them the default
    `*:80`, producing a flood of "conflicting server name"
    warnings and attaching every server block to an unrelated
    wildcard bind;
  * the bindings list grew linearly with the number of server
    blocks, so init_module tried to create (server_blocks) ×
    (devices × families) listening sockets and hit EMFILE.

Replace the single dedup with two independent checks:

  * listens_seen is a (cscf, sockaddr) ledger. The core listen
    handler is invoked at most once per (server block, sockaddr),
    matching nginx's own duplicate check so server-block N just
    attaches its cscf to the existing address via
    ngx_http_add_server.

  * `bind` is added only for the first global occurrence of each
    sockaddr; subsequent cscfs inherit opt.set/opt.bind from the
    first, which is what keeps nginx's "duplicate listen options"
    check happy across server blocks.

  * bindings dedup on (sockaddr, device) globally, so init_module
    creates one socket per unique pair regardless of how many
    server blocks reference it.

Add a regression test at tests/01-module/ that wires three server
blocks to the same ipng-listens.inc and asserts that nginx -t is
clean and exactly four sockets are bound on port 8080.

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

tests/01-module/01-e2e.robot

@@ -29,6 +29,23 @@ Module loads
     ${output} =    Docker Exec    ${SERVER}    nginx -t 2>&1
     Should Contain    ${output}    syntax is ok
 
+Shared-listen-include across multiple server blocks
+    [Documentation]    Three server blocks all pull in the same
+    ...                ipng-listens.inc (see docs/user-guide.md). nginx
+    ...                must start without "conflicting server name" or
+    ...                "duplicate listen options" warnings, and the
+    ...                module must end up with exactly one listening
+    ...                socket per (device, family) pair — not one per
+    ...                (server block × device × family), which would
+    ...                exhaust the fd table on a real host.
+    ${output} =    Docker Exec    ${SERVER}    nginx -t 2>&1
+    Should Not Contain    ${output}    conflicting server name
+    Should Not Contain    ${output}    duplicate listen
+    ${listens} =    Docker Exec    ${SERVER}    ss -tlnH
+    ${count} =    Get Regexp Matches    ${listens}    :8080\\s
+    Length Should Be    ${count}    4
+    ...    Expected 4 listening sockets on port 8080 (v4+v6 × eth1+eth2); got ${count}
+
 Prometheus scrape
     [Documentation]    Scrape returns HELP/TYPE preamble.
     ${output} =    Scrape Prometheus