[Vulcan] Clarify parsed-part mutation and fixed dictionary rendering

[iliaal]

Summary

• Correct the AbstractPart::setValue() comment that incorrectly described all mapped parts as frozen. Ordinary name parts invalidate their camelcase cache, and initials read their current value; only pre-normalized dictionary parts retain constructor rendering.
• Document how to edit parsed parts, replace dictionary-rendered objects with Name::setParts(), and retain awareness that part edits do not update source or rerun parsing.
• Documentation only: executable PHP token stream is unchanged. No parser output changes and no changelog runtime claim.

Verification

• PHP 8.4: full PHPUnit suite passed (1,242 tests, 3,271 assertions).
• PHPStan level 9 and PHP 8.5 compatibility configuration passed with a 512 MiB memory limit; the local default 128 MiB was insufficient.
• PHP CS Fixer dry run, empty-baseline guard, PHP 8.5 deprecation grep, README em-dash check, and git diff --check passed.
• Runtime probes checked cached Smith → Jones, current-value initials, fixed salutation/suffix/surname-prefix/middle-prefix rendering after raw mutation, replacement suffix PhD, shared getParts() objects, and unchanged original source.
• Independent review verified all claims against the existing implementation and swept related docs/comments for contradictions.
• Complete remote tree 34110f3310246d3c02a3c234f2f08c94982a2ee1 matches the tested local tree, including every file and mode.

No tests were added because this corrects documentation of existing behavior; the existing cache-invalidation and partner-isolation tests remain unchanged. PHP 8.3/8.5 and repository CI will be verified on this exact head.