test: cover migrated centrality, community, and centralization signatures

[krlmlr]

Argument-coverage tests for the 25 functions migrated in the already-merged
#2763 (centrality), #2765 (community), and #2764 (centralization).

For every migrated function:

• each keyword-only tail argument is exercised by name with a
  non-default value and a value-based assertion (exact scores, partition
  invariants, algebraic identities such as the resolution identity
  B(2) = 2·B(1) − A and exo linearity);
• a legacy positional recovery test proves the old call form still works
  and emits the soft deprecation
  (lifecycle::expect_deprecated() + equality against the named call).

Tally for centrality|community|centralization: FAIL 0 · WARN 0 · PASS 2708
(one new snapshot in _snaps/community.md).

Notes:

• modularity_matrix(): a working legacy positional recovery is impossible
  (its first old slot is the hard-deprecated membership); covered with an
  error snapshot capturing both the recovery warning and the membership
  error.
• Stochastic cluster_* functions use igraph_local_seed() with seeded
  expect_identical() recovery comparisons.
• centr_betw_tmax()/centr_clo_tmax() cover both the graph= and the
  graph = NULL + nodes= forms, cross-checked against
  centr_*()$theoretical_max.
• Weighted cluster_edge_betweenness() emits an igraph C warning when
  selecting a membership; asserted explicitly so the suite stays
  warning-clean.

Part of the lifecycle/coverage exercise on top of the #2782 baseline.

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