Revert auto-assigned ids back to letters; only detected ids are numeric

Auto-assignment (no real id known: a manual add, or an accepted
proposal with no confident marker-id read) must stay visually
distinct from a genuinely detected id, or a made-up number could
collide with or be mistaken for a real one. Reverts the previous
commit's switch to numeric auto-assignment (_next_free_numeric_id) --
that was wrong, caught by the user immediately. _next_free_id
(letters, rolling over to "AA"/"AB"/... past 26) is back as the
fallback, still scoped per type (that part of the previous change was
correct and stays). Plain numbers are reserved for an id
_accept_proposal is actually confident was read off the marker itself
(Proposal.detected_id), passed straight through and never touching
auto-assignment.

Also fixes detected_id's own collision pre-check in _accept_proposal,
which wasn't scoped per type either -- same bug as the Change ID
popover fix, just in a second place: a detected id could get
needlessly discarded because an unrelated type already used that
number, not because of a real collision.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_models.py

@@ -54,58 +54,60 @@ def test_clear_units_keeps_recon_infrastructure():
 def test_ally_and_target_ids_are_independent_namespaces():
     """An ally Tank#1 and a hostile Target Tank#1 are unrelated -- adding
     one must never be influenced by the other's ids, and auto-assignment
-    on each side starts from '1' independently."""
+    on each side starts from 'A' independently. Explicit id_="1" here
+    (as an accepted screenshot proposal's detected_id would pass, see
+    app.py's _accept_proposal) to also check that auto-assignment
+    correctly skips a real numeric id already in use, not just other
+    letters."""
     board = Board()
     t1 = board.add_target(TargetType.TANK, _coord(), id_="1")
     a1 = board.add_ally(TargetType.TANK, _coord(), id_="1")
     assert t1.id == a1.id == "1"
     assert t1 is not a1
 
-    # '1' is already taken (explicitly) for TANK on each side, so the next
-    # auto-assigned Tank on each side must skip it and land on '2'.
     t_auto = board.add_target(TargetType.TANK, _coord())
     a_auto = board.add_ally(TargetType.TANK, _coord())
-    assert t_auto.id == "2"  # first free number among *target* Tanks only
-    assert a_auto.id == "2"  # first free number among *ally* Tanks only, unaffected by the target above
+    assert t_auto.id == "A"  # first free letter among *target* Tanks only
+    assert a_auto.id == "A"  # first free letter among *ally* Tanks only, unaffected by the target above
 
 
 def test_auto_id_is_per_type_within_targets_and_within_allies():
-    """Each TYPE gets its own independent 1/2/3... sequence within a group
+    """Each TYPE gets its own independent A/B/C... sequence within a group
     (all targets, or all allies) -- a Tank and an Infantry auto-assigned
-    back to back both start at '1' (Tank#1, Infantry#1), matching how the
-    game itself numbers units, rather than sharing one sequence across
-    every type in the group."""
+    back to back both start at 'A' (Tank#A, Infantry#A), rather than
+    sharing one sequence across every type in the group."""
     board = Board()
     tank = board.add_target(TargetType.TANK, _coord())
     infantry = board.add_target(TargetType.INFANTRY, _coord())
-    assert tank.id == "1"
-    assert infantry.id == "1"  # own sequence, not '2' just because a Tank came first
+    assert tank.id == "A"
+    assert infantry.id == "A"  # own sequence, not 'B' just because a Tank came first
 
     second_tank = board.add_target(TargetType.TANK, _coord())
-    assert second_tank.id == "2"  # but a SECOND Tank does advance the Tank sequence
+    assert second_tank.id == "B"  # but a SECOND Tank does advance the Tank sequence
 
     ally_tank = board.add_ally(TargetType.TANK, _coord())
     ally_infantry = board.add_ally(TargetType.INFANTRY, _coord())
-    assert ally_tank.id == "1"
-    assert ally_infantry.id == "1"
+    assert ally_tank.id == "A"
+    assert ally_infantry.id == "A"
 
 
-def test_auto_id_keeps_counting_past_26_entities_of_one_type():
-    """The id sequence is a plain integer counter now (see
-    _next_free_numeric_id), not the old letter sequence that could raise
-    StopIteration past 26 (see TODO.md/git history) -- nothing special
-    should happen crossing 26, it just keeps counting."""
+def test_auto_id_survives_past_26_entities_of_one_type():
+    """A real crash: `next(c for c in string.ascii_uppercase if c not in
+    used)` raises StopIteration the instant all 26 letters are taken --
+    reachable after accepting/adding 26+ of the same type into one group
+    in a single session. Must roll over to two-letter ids ('AA', 'AB',
+    ...) instead of raising."""
     board = Board()
     for _ in range(26):
         board.add_target(TargetType.TANK, _coord())
     twenty_seventh = board.add_target(TargetType.TANK, _coord())
-    assert twenty_seventh.id == "27"
+    assert twenty_seventh.id == "AA"
 
     board2 = Board()
     for _ in range(26):
         board2.add_ally(TargetType.TANK, _coord())
     twenty_seventh_ally = board2.add_ally(TargetType.TANK, _coord())
-    assert twenty_seventh_ally.id == "27"
+    assert twenty_seventh_ally.id == "AA"
 
 
 def test_find_by_name_prefers_target_over_same_named_ally():