add connection-scoped context navigation

docs/plugin-api-additions/contexts.md

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+# Connection and context navigation
+
+`get_connections()` returns one Connection snapshot per live connection:
+`id`, `network`, `server`, `context`. Connection IDs distinguish simultaneous
+connections with identical names. Prefer its server-tab context where present.
+`get_connection(context=None)` finds the owner of a live context.
+
+`get_contexts(connection_id=None, types=None)` lists live contexts, optionally
+filtered by connection ID (or a Connection object) and session types: server=1,
+channel=2, query=3. `connection.contexts(types=None)` is a convenience method.
+`find_context_by_id(connection_id, channel=None)` / `connection.find(channel)`
+resolve using that connection's IRC comparator; no channel selects the server tab.
+Missing/closed contexts return None. Existing find_context() remains unchanged.
+
+These helpers use the existing channels list's ID/context fields, so no new C
+context ABI is needed. Snapshots are not persistent IDs across client restarts;
+refresh them after closing/recreating connections. Always check Context.set()
+when explicitly selecting a context that might have closed.
+
+```python
+connection = zoitechat.get_connection()
+for context in connection.contexts(types=(2,)):
+    context.prnt('A channel on this exact connection')
+```

plugins/python/_zoitechat_contexts.py

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+"""Navigate live connections by ID instead of ambiguous server/network names."""
+from collections import namedtuple
+import operator
+import _zoitechat as api
+
+__all__ = ['Connection', 'get_connections', 'get_connection', 'get_contexts',
+           'find_context_by_id']
+
+
+class Connection(namedtuple('_Connection', 'id network server context')):
+    __slots__ = ()
+
+    def contexts(self, types=None):
+        return get_contexts(self.id, types)
+
+    def find(self, channel=None):
+        return find_context_by_id(self.id, channel)
+
+
+def _id(connection):
+    return operator.index(connection.id if isinstance(connection, Connection) else connection)
+
+
+def get_connections():
+    """Return one snapshot per live connection, including disconnected ones."""
+    entries = api.get_list('channels')
+    by_id = {}
+    order = []
+    for entry in entries:
+        if entry.id not in by_id:
+            order.append(entry.id)
+        if entry.id not in by_id or entry.type == 1:
+            by_id[entry.id] = Connection(entry.id, entry.network, entry.server,
+                                         entry.context)
+    return [by_id[connection_id] for connection_id in order]
+
+
+def get_connection(context=None):
+    """Find the owning connection of a Context; return None when it has closed."""
+    context = context or api.get_context()
+    for entry in api.get_list('channels'):
+        if entry.context == context:
+            for connection in get_connections():
+                if connection.id == entry.id:
+                    return connection
+    return None
+
+
+def get_contexts(connection_id=None, types=None):
+    """Return contexts, optionally restricted to a connection and session types."""
+    if connection_id is not None:
+        connection_id = _id(connection_id)
+    if types is not None:
+        types = frozenset(types)
+    return [entry.context for entry in api.get_list('channels')
+            if (connection_id is None or entry.id == connection_id)
+            and (types is None or entry.type in types)]
+
+
+def find_context_by_id(connection_id, channel=None):
+    """Resolve a live context on exactly one connection; None selects its server tab."""
+    connection_id = _id(connection_id)
+    previous = api.get_context()
+    try:
+        for entry in api.get_list('channels'):
+            if entry.id != connection_id:
+                continue
+            if channel is None:
+                if entry.type == 1:
+                    return entry.context
+            elif entry.context.set() and api.nickcmp(entry.channel, channel) == 0:
+                return entry.context
+    finally:
+        previous.set()
+    return None