docs(perf): record measured CLI and GixPython repository journeys

Document the new `pyperf` workload and local measurements independently of
full-suite setup/coverage timings. Record all eleven means and standard
deviations, fixture/source revisions and environment details: direct opening
and nested discovery favor GixPython, but the already-open journey is about
13% slower because history metadata and commit statistics offset other gains.

Explain retained `Repo` ownership during operation measurements, separate
open/discovery lifecycle costs, fresh high-level wrappers, parity checks and
the extensible CI artifact workflow. Native handle reuse remains future work;
these measurements expose its potential benefit without implementing it.

Both complete calibrated runs finished on existing CPython 3.12.14 with
GixPython first and CLI second. All eleven result digests match; the comparison
and `pyperf` significance table completed successfully. Record the stability
warnings rather than treating one machine's warm-cache values as universal.
The initial dedicated CI benchmark job also passed. `git diff --check` passed.

Author: codex

doc/gix-backend.md

@@ -118,6 +118,54 @@ only an absent top-level `gix` selects CLI mode.
 
 ## Performance work
 
+### Existing-repository benchmark
+
+[`test/performance/README.md`](../test/performance/README.md) describes the
+`pyperf` harness and fixed GitPython 3.1.45 fixture. It measures eight public-API
+operations and their complete journey on one already-open `git.Repo`, plus
+separate direct opening and discovery from `git/objects`. Fresh high-level
+wrappers preserve the cost of actual operations; imports, fixture preparation
+and parity preflight are outside timing. Native repository reopening remains
+inside operation timing, so retaining native state in the future can improve
+these measurements without changing the workload.
+
+At `244e418da6cc43de129cbe2908d11be4c5ad457a`, using official GixPython
+0.1.0 and the same existing CPython 3.12.14/macOS arm64 interpreter for both
+installations, with Git 2.54.0 (Apple Git-157), six worker processes with three values each produced the
+following means and standard deviations. GixPython ran first, then CLI;
+all eleven result digests matched. The fixture has one branch, one untracked
+file and an ignored directory, with ambient Git configuration disabled.
+
+| Measurement | CLI (ms) | GixPython (ms) | CLI / Gix |
+| --- | ---: | ---: | ---: |
+| Already-open journey | 230.90 ± 6.87 | 260.14 ± 13.37 | 0.89× |
+| Reference inventory | 22.77 ± 1.15 | 1.17 ± 0.26 | 19.52× |
+| 25 first-parent commits with metadata | 9.24 ± 0.46 | 73.61 ± 7.24 | 0.13× |
+| Root tree and README blob | 16.02 ± 1.07 | 2.40 ± 0.46 | 6.66× |
+| Index entries | 8.43 ± 0.57 | 2.00 ± 0.38 | 4.21× |
+| Commit count and ancestry queries | 82.28 ± 2.27 | 36.19 ± 0.84 | 2.27× |
+| Latest diff and commit statistics | 29.98 ± 1.66 | 60.98 ± 2.45 | 0.49× |
+| Patch diff (includes CLI fallback) | 17.87 ± 1.28 | 10.86 ± 1.01 | 1.64× |
+| Dirty/untracked/ignored paths | 68.56 ± 3.18 | 64.74 ± 2.00 | 1.06× |
+| Direct opening and close | 95.08 ± 4.64 | 35.84 ± 2.02 | 2.65× |
+| Nested discovery and close | 112.58 ± 5.38 | 49.20 ± 4.16 | 2.29× |
+
+Ratios above one favor GixPython. This workload's complete journey is about
+13% slower with GixPython: metadata reads and commit statistics offset gains
+elsewhere. These are warm-cache measurements on one machine, with `pyperf`
+stability warnings for several samples; they do not establish a general
+speedup. Full-suite times below also include setup and coverage.
+
+The separate `Backend benchmark` CI job runs both installations on one runner,
+publishes every measurement and `pyperf` significance reporting, and retains
+raw JSON and comparison artifacts. Results include revisions and per-operation
+native/fallback decisions. Adding a `MEASUREMENTS` entry extends the journey,
+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.
+
+### Test-suite setup measurements
+
 Native repository reuse remains future work. The fixture optimizations below
 are implemented as separate commits, each validated with GixPython before CLI.
 The original local measurements use official GixPython 0.1.0 and existing