🐛 Don't mutate rebuild='env' src_trace_projects config during builds

[ubmarco]

Summary

Incremental Sphinx builds always re-read every document, logging
[config changed ('src_trace_projects')] N added, 0 changed, 0 removed, even
when nothing changed — incremental builds behave like full rebuilds.

src_trace_projects is registered with rebuild="env", so Sphinx pickles it
into environment.pickle and invalidates the environment whenever the pickled
("old") value differs from the freshly computed ("new") one.
generate_project_configs() stores runtime analyse_config /
source_discover_config dataclass objects in that value; on their own they
round-trip through pickle as ==-equal.

The bug: the src-trace directive mutated the stored analyse_config in
place during the build (src_dir, src_files, git_root set to build-time,
resolved values). Those build-time values were persisted into the pickle, while
the next build's config-inited produced a fresh analyse_config
(src_files=[], src_dir=Path(".")), so the comparison always differed:

Side	src_dir	src_files
persisted (pickle, "old")	/abs/.../dcdc	['demo_3.cpp', ...]
fresh config-inited ("new")	Path('.')	[]

Fix

Operate on a per-directive dataclasses.replace() copy instead of mutating the
analyse_config stored in the rebuild="env" config value. The stored value
then stays equal to what generate_project_configs() yields, so incremental
builds report 0 added, 0 changed, 0 removed.

Verification

• New regression test test_incremental_build_keeps_src_trace_projects_unchanged
  builds the fixture twice and asserts the 2nd (incremental) build sees
  CONFIG_OK — fails before the fix (CONFIG_CHANGED ('src_trace_projects')),
  passes after.
• Full suite: 207 passed.
• Manual triple build: builds 2 & 3 now report 0 added, 0 changed, 0 removed.

Fixes #74

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.86%. Comparing base (c9e78da) to head (ef25f45).

Files with missing lines	Patch %	Lines
...codelinks/sphinx_extension/directives/src_trace.py	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
+ Coverage   90.79%   90.86%   +0.06%     
==========================================
  Files          30       30              
  Lines        2771     2790      +19     
  Branches      305      305              
==========================================
+ Hits         2516     2535      +19     
  Misses        160      160              
  Partials       95       95              

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