Make Visibility enum string-backed

[Firehed]

Closes #295.

Visibility was int-backed solely to make isAccessibleFrom a >= comparison, despite significant adjacent string handling (format(), fromString(), and a raw ->value max() in TextFallbackHelper).

Changes

• Flip the backing from int to string ('private'/'protected'/'public'). Accessibility ordering now lives in a private rank().
• Add Visibility::atLeast(), replacing the raw Visibility::from(max($minVisibility->value, Visibility::Protected->value)) arithmetic in the text fallback.
• format() collapses to return $this->value;, removing the duplicated literals.
• Delete Visibility::fromString() in favor of native Visibility::from(). The three call sites parse a regex capture group that only ever matches the lowercase public|protected|private literals, which are now exactly the enum values; the empty-constant case keeps its existing explicit ternary.

The fromString test was removed alongside the method it covered.

🤖 Generated with Claude Code

[Firehed]

this actually may slightly decrease robustness by removing the lowercase match or pushing it to the edges, leaving it alone for the moment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.78%. Comparing base (6f603d2) to head (549a0af).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #354      +/-   ##
============================================
- Coverage     96.78%   96.78%   -0.01%     
+ Complexity     1625     1624       -1     
============================================
  Files            97       97              
  Lines          4076     4074       -2     
============================================
- Hits           3945     3943       -2     
  Misses          131      131              

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