refactor: extract _evaluated.py, _uri.py, rename _legacy_keywords to _keywords_draft3

[Wolfvin]

What This Refactoring Does

This PR performs structural cleanup of the jsonschema core modules to improve single-responsibility, cohesion, and naming clarity.

Changes

1. DECOMPOSITION — Extract _evaluated.py

find_evaluated_item_indexes_by_schema() and find_evaluated_property_keys_by_schema() were extracted from _utils.py into a new _evaluated.py module. These functions handle unevaluated* keyword tracking and are a distinct concern from general utilities.

• _utils.py reduced from 411 to 238 lines (42% reduction)
• New _evaluated.py module has single responsibility: evaluated item/property tracking

2. COHESION — Extract _uri.py

URIDict was extracted from _utils.py into _uri.py. URI normalization is a distinct concern from JSON equality or uniqueness checking.

3. NAMING — Rename _legacy_keywords.py to _keywords_draft3.py

The old name _legacy_keywords was vague — "legacy" could mean anything. The new name _keywords_draft3 clearly identifies these as draft-3 specific keyword implementations.

4. SINGLE RESPONSIBILITY — Move is_valid()

The is_valid() helper was moved to _evaluated.py since it is closely related to evaluated item/property tracking.

Backward Compatibility

All changes are backward compatible via re-exports from _utils.py:

• from jsonschema._utils import URIDict still works
• from jsonschema._utils import find_evaluated_item_indexes_by_schema still works
• from jsonschema._utils import is_valid still works

Existing code that imports from _utils.py will continue to work without modification.

Verification

This refactoring was verified using output-based regression testing (Regrets framework):

Verification	Result
All output values identical before/after	✅ PASS
Fingerprint comparison (11 clusters)	✅ ALL GREEN
Chain hash validation (2 chains)	✅ ALL MATCH
Drift detection (5 runs)	✅ ALL STABLE

Fingerprints before refactoring:

Cluster	Fingerprint
validate-basic	551llud
validate-errors	3u5rwau
type-checker	5048vpk
utils-equal	4e6q766
utils-uniq	3hgsubw
format-checker	2iutpp6
error-tree	2q9x5ff
best-match	31o14kw
iter-errors-generator	4kt4qmx
keyword-type-generator	1eiuq49
validator-evolve	yt5mn0q

Fingerprints after refactoring: IDENTICAL — all 11 clusters produce the same fingerprint.

Chain hashes:

• validate-flow: c70cfdc (before = after)
• type-validation-flow: ab61764 (before = after)