docs(gix): track strict Git compatibility and verified measurements

Treat observed differences from equivalent Git operations as compatibility
bugs, even when intentional or covered by an adapter workaround. Require an
upstream fix or a verified API/mode as strict as Git. Define `native` as
Gitoxide execution through GixPython, name the underlying Gix calls, and forbid
Python implementations of missing Git behavior. Keep reproduced bugs, missing
APIs and unverified coverage distinguishable in the follow-up ledger.

The ledger now records 25 items. Add object/branch type validation (`GIX-24`)
and dangling-reference enumeration (`GIX-25`), expand identity cleanup
(`GIX-21`), and spell out the missing HEAD reflog updates and incorrect old OID
on symbolic detachment (`GIX-10`). Preserve bare-worktree classification and
canonical gitfile behavior as upstream compatibility bugs (`GIX-14`) even though
the adapter obtains compatible results from Gix metadata and reopening.
Include Git/Gix expectations, API names, reproduction instructions, versions
and the existing guard regressions. Keep current CLI fallbacks and budgets.

Verify 12 direct Gix/Git comparisons for each of SHA-1 and SHA-256 on released
GixPython 0.1.0 and existing CPython 3.12.14: six object/branch type checks,
identity cleanup, dangling enumeration and four reflog transactions. Gix ran
before Git for each case. All 24 comparisons confirmed the documented outcomes;
ledger IDs, status labels, cross-references and local links are consistent.

Preserve the earlier Gix-only audit measurements at `5ef5c62f`: the 1,710-test
full suite, 22,694 CLI launches, and 14 matching `pyperf` result digests. Keep
the subsequent 120-Gix/34-CLI-test capability validation tied to its runtime
snapshot `ee947c98`. The historical zero-launch lifecycle results are not
current coverage claims. This documentation follow-up reruns neither the full
suite nor timed benchmarks and does not claim those measurements for a new
runtime revision.

This commit reduces CLI launches by zero. Runtime code, tests and CLI budgets
are unchanged by the strict-compatibility documentation update. All 74 original
Tix changes remain represented, and `git diff --check` passes. No additional
interpreter was installed.

Author: codex