feat: ProfileConfig.string_sentinels declaration + StructuralProfileResult propagation

[DEVunderdog]

Parent

#176

What to build

Add the user-facing string sentinel declaration surface to ProfileConfig and thread it through to StructuralProfileResult so Phase 2 can read declared sentinels from the profile without holding a config reference.

ProfileConfig (profiling/_config.py):

• Add string_sentinels: dict[str, list[str]] field with default empty dict
• Update to_dict() to serialize the field
• Update from_dict() to restore it (missing key → empty dict, for backwards compatibility)
• Update the class docstring to document the new parameter

StructuralProfileResult (profiling/_config.py):

• Add string_sentinels: dict[str, list[str]] field with default empty dict
• StructuralProfiler.profile() copies profile_config.string_sentinels into the result when building it

Acceptance criteria

• ProfileConfig.string_sentinels field exists, defaults to {}
• ProfileConfig.to_dict() includes the string_sentinels key
• ProfileConfig.from_dict() restores string_sentinels; a dict without the key defaults to {}
• StructuralProfileResult.string_sentinels field exists, defaults to {}
• StructuralProfiler.profile() copies profile_config.string_sentinels onto the returned StructuralProfileResult
• A ProfileConfig with no sentinels declared serializes and deserializes identically to the current behaviour
• Unit tests cover: round-trip with sentinels declared, round-trip with no sentinels, old-format dict without the key deserializes to empty dict, profiler propagates the dict to the result
• Docstrings updated per ADR-0034

Blocked by

None — can start immediately