docs(perf): preserve pre-audit retained-handle measurements

Record the original retained-handle/opening benchmark, full-suite timings,
launch counts and validation with their measured revision and local artifacts.
These observations predate the Gix-only audit and include the Python
reference-format, linked-worktree and negative-discovery shortcuts that the
rewritten opening commit removes.

Label the zero-call opening/discovery rows and their 69x/91x ratios as
historical, superseded measurements. They must not be read as current Gix
coverage or current performance. Keep the already-open journey and test
observations alongside their original context rather than silently replacing
old data with new claims.

This documentation amendment reduces CLI calls by zero and changes no runtime
behavior. Validation: `git diff --check` passed; the corrected opening commit
already passed its focused Gix tests followed by CLI tests.

Author: codex

doc/gix-backend.md

@@ -212,6 +212,63 @@ individual timings and parity checks together. Opening/discovery remain
 separate lifecycle measurements. CI fails on execution/parity errors; timing
 ratios are observational on shared runners.
 
+### Historical retained-handle and opening measurements
+
+These results predate the Gix-only audit. The measured implementation inferred
+reference format, repaired linked-worktree metadata and rejected some discovery
+candidates in Python. Its zero-CLI counts and associated opening speedups do
+not establish Gix coverage and are superseded by the corrected implementation.
+Keep the raw observations below only as a historical record.
+
+At `c49bbec0afb806df66a33fa9c5d76e21ac8f0314`, the same pinned fixture,
+official GixPython 0.1.0 and existing CPython 3.12.14/macOS arm64 interpreter
+were measured sequentially, Gix first and CLI second, after tests finished.
+Each backend used three worker processes, three values per worker, four loops
+per value and one warmup. All eleven result digests matched and the checked-in
+CLI ceilings passed. Raw local results are
+`/private/tmp/gitpython-retained-final-{gix,cli}.json`; this table is a journal
+snapshot, while CI continues to publish fresh measurements.
+
+| Measurement | CLI mean ± stdev (ms) | GixPython mean ± stdev (ms) | CLI / Gix | Native / fallback | CLI launches: CLI / Gix |
+| --- | ---: | ---: | ---: | ---: | ---: |
+| journey | 209.44 ± 17.65 | 263.25 ± 5.52 | 0.80× | 55 / 2 | 23 / 1 |
+| inventory | 19.76 ± 0.78 | 0.89 ± 0.03 | 22.30× | 3 / 0 | 3 / 0 |
+| history_25 | 7.49 ± 0.17 | 79.02 ± 9.67 | 0.09× | 26 / 0 | 1 / 0 |
+| browse_tree_and_blob | 13.55 ± 0.39 | 2.06 ± 0.41 | 6.59× | 5 / 0 | 2 / 0 |
+| read_index | 6.96 ± 0.08 | 2.59 ± 0.47 | 2.69× | 1 / 0 | 1 / 0 |
+| revision_graph | 63.27 ± 2.57 | 32.72 ± 0.22 | 1.93× | 9 / 0 | 6 / 0 |
+| diff_and_stats | 20.78 ± 0.37 | 65.33 ± 1.25 | 0.32× | 5 / 0 | 3 / 0 |
+| patch_diff | 14.06 ± 0.39 | 8.61 ± 0.08 | 1.63× | 3 / 2 | 2 / 1 |
+| worktree_status | 56.61 ± 1.43 | 70.68 ± 2.18 | 0.80× | 3 / 0 | 5 / 0 |
+| open_repository | 69.28 ± 0.59 | 1.00 ± 0.07 | 69.20× | 1 / 0 | 11 / 0 |
+| discover_repository | 81.68 ± 0.68 | 0.90 ± 0.19 | 90.52× | 1 / 0 | 13 / 0 |
+
+The pre-audit implementation launched no CLI processes for these opening and
+discovery probes. It measured about 69× and 91× faster lifecycle operations,
+while the complete already-open journey was about 26% slower with Gix. These
+lifecycle ratios include the shortcuts removed by the audit. Retaining a Python
+handle alone does not resolve the existing history/statistics costs. Several
+rows have `pyperf` sample-size or variability warnings, so timings describe
+this machine and workload rather than a general speed guarantee.
+
+The full Gix suite without coverage passed **1,678 tests and 38 subtests**,
+with 79 skips and one expected failure, in **337.51 seconds (5m 37.51s)**.
+One expected path-expansion deprecation warning was reported. It recorded
+**22,319 `Git.execute` launches**: 3,833 setup, 18,486 call, zero teardown or
+collection/session. The process-lifetime count is 22,320 including the import
+probe. Against the preceding 408.81-second/30,898-launch baseline, this local
+run took 17.4% less time and launched 27.8% fewer CLI processes; the suite has
+four additional regressions, and the timing comparison is observational.
+
+Affected tests ran with Gix before CLI: 304/264 repository, command, backend
+and safety tests; 136 command-guard tests per backend; 31 positional tests per
+backend; 13 process-count/budget tests per backend. Repository suites also
+passed 14 subtests, with three skips. Ruff lint/format, mypy and basedpyright
+passed. Both code changes are separate Tix commits: `02e2cc44` retains native
+handles and the original `c49bbec0` attempted zero-launch opening/discovery.
+The rewritten opening commit restores real format queries and native metadata
+fallbacks; the current coverage table and ceilings reflect those requirements.
+
 ### Test-suite setup measurements
 
 The fixture optimizations below are implemented as separate commits, each