Recover attribution after nuanced Trace2 gaps - #2074

Summary

• add TestRepo coverage for commit, amend, reset, rebase, pull merge/rebase, merge, squash merge, cherry-pick, revert, stash, branch force, and update-ref recovery after missing Trace2 events
• test multi-commit and multi-source spans, rename preservation, symbolic stash targets, stdin ref transactions, linked worktrees, and duplicate reflog history
• select the current pull-rebase span using its captured ingress boundary
• reconcile symbolic stash targets with the command-start stash tip after missed stack mutations
• process branch force rewrites through the same exact RefUpdated path as update-ref
• bound unstructured update-ref transactions to the current command so missed transactions remain unattributed
• keep recovery in memory over already captured refs and reflogs; no new Git calls or ingestion-path I/O

Test plan

• task test CARGO_TEST_ARGS="--test integration" (3292 passed, 89 ignored)
• task test CARGO_TEST_ARGS="--test commit_tree_update_ref" (39 passed, 2 ignored)
• task lint
• task fmt

Depends on #2073.

Stack: 2 of 2.