fix(zones): a West channel no longer receives the unmarked East feed (#55)

[PiratesIRC]

Fixes #55.

What was wrong

extract_zone reads a stream name with no zone marker as DEFAULT, which its docstring calls the East feed in a US lineup, and the channel side already routes an unmarked channel as East. _order_streams_for_zone dropped only a stream literally marked EAST from a West channel, so the same feed was dropped when written INVESTIGATION CHANNEL (EAST) and kept when written INVESTIGATION CHANNEL.

Measured on the maintainer's install: STARZ (W) held 3 West rows and 6 unmarked rows, Showtime (W) held 9 West rows and 6 unmarked rows.

What changes

• A West channel keeps West streams only. _zone_affinity_rank ranks unmarked equal to East on a West channel.
• When no West stream matched, Match and Assign and Preview now use _streams_to_assign:
  • the channel already holds streams: nothing is added and nothing is removed, in both Overwrite modes (appending would add the wrong feed; replacing with an empty list would take the channel off the air);
  • the channel holds no streams: the other-zone streams are assigned, with a logged warning, as before.
• Sort keeps the keep-everything fallback, because it deletes and recreates a channel's rows from the list.
• A lone West-named channel with no sibling is still not zone-routed, so it is unaffected.

Tests

tests/test_issue_55_west_unmarked.py (13 tests), plus four existing tests rewritten because they asserted the old behaviour. Full suite: 1547 passed. Each guard was removed in turn and a named test failed each time; a comment-only change left the suite green.

Effect on the maintainer's install once deployed

The daily Sort will remove the 12 unmarked rows from STARZ (W) and Showtime (W); both keep their West streams. No other West channel is affected.

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