add trace pruning collapsing cross-service (PR-TRACE-1b) (#245)

* add trace pruning, collapsing, cross-service (PR-TRACE-1b)

Implements four pruning features for the trace BFS engine:
- prune_roles soft gate (edges recorded, frontier stops)
- fan_out_cap with confidence + role ranking (scaffolding exempt)
- collapse_trivial post-BFS pass with parent_edge_id recomputation
- cross-service boundary detection (DECLARES_CLIENT/PRODUCER → HTTP/ASYNC_CALLS)

Replaces 1a's test_trace_prune_roles_param_accepted_noop stub with
test_trace_filter_vs_prune_roles. 10 additional 1b tests. 33 total
unique tests (31 pass, 2 skip on fixture gaps). Full suite green (648
passed). No changes to mcp_v2.py, kuzu_queries.py, server.py.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: address review feedback on PR-TRACE-1b (#245)

- Fix vacuous assertion in test_trace_cross_service_edge_attrs
- Add docstring to _collapse_trivial_chains noting single-pass and mutation
- Add stderr logging to cross-service exception handlers

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix: remove duplicate continue in cross-service exception handler

Dead code introduced in review fix commit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: HumanBean17